---------------------------------------------------------- · – continental extensional tectonics – the paparoa metamorphic core complex of westland, new zealand. a thesis submitted

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– CONTINENTAL EXTENSIONAL TECTONICS –

THE PAPAROA METAMORPHIC CORE

COMPLEX OF WESTLAND, NEW ZEALAND.

A THESIS SUBMITTED IN PARTIAL

FULFILLMENT OF THE REQUIREMENTS FOR

THE DEGREE OF

MASTER OF SCIENCE IN GEOLOGY

AT THE

UNIVERSITY OF CANTERBURY

BY

MICHELLE ERICA JUNE HERD

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UNIVERSITY OF CANTERBURY

2007 ----------------------------------------------------------------------------------------------------------------------------

Frontispiece:

– Ductile deformation associated with the Pike Detachment Fault –

ultra-mylonite outcropping at Morrisey Creek, north of Fox River Mouth.

TABLE OF CONTENTS List of figures p. 1 List of tables p. 10 Acknowledgements p. 15 Abstract p. 16 CHAPTER 1: INTRODUCTION p. 17

1.1. Metamorphic core complexes p. 17 1.2. Regional geology p. 32 1.3. Thesis objectives p. 39 1.4. Location of study p. 41 1.5. Methodology p. 42

1.5.1. Fieldwork p. 42 1.5.2. Geochemical work: CL imaging, SIMS and LA-ICP-MS p. 43 1.5.3. Geochemical work: whole rock analysis (XRF) p. 46

CHAPTER 2: STRUCTURAL GEOLOGY p. 47

2.1. Previous work p. 47 2.2. The Paparoa Metamorphic Core Complex p. 52 2.3. Own observations p. 59 2.4. Hand specimens p. 63 2.5. Thin section work p. 65 CHAPTER 3: ZIRCONOLOGY p. 79 3.1. Use of zircon in geochronology p. 79 3.2. Zircon morphology p. 81 3.2.1. Crystal shape p. 81 3.2.2. Primary oscillatory growth zoning p. 82 3.2.3. Alteration and metamorphism p. 83 3.2.4. Pb-loss p. 84

3.3. Morphology of zircons from the PMCC p. 85 3.3.1. RNZ119 p. 91 3.3.2. UC08368 p. 93 3.3.3. PCC06-01 p. 95 3.3.4. UC05592 p. 96

3.4. Discussion of PMCC zircon morphology p. 98 CHAPTER 4: GEOCHEMISTRY AND GEOCHRONOLOGY p. 102

4.1. Geochemical investigations p. 102 4.1.1. Whole rock analysis (XRF) p. 102 4.1.2. Oxygen isotopes p. 103 4.1.3. Titanium p. 108 4.1.4. Hafnium p. 116 4.1.5. REE’s p. 124

4.2. Geochronological investigations p. 144 4.2.1. U-Pb dating p. 144

4.3. Combined geochemistry and geochronology p. 157 4.4. Provenance of zircon grains p. 169

4.4.1. Provenance of zircons from the PMCC p. 170

CHAPTER 5: DISCUSSION p. 172 5.1. Geometry and nature of metamorphism of the PMCC p. 172 5.2. Geochemical and geochronological findings p. 175 5.3. Potential future work p. 184

CHAPTER 6: CONCLUSION p. 186 REFERENCES p. 190

1

LIST OF FIGURES Figure 1.1………………………………………………………………………….p. 23

Model of metamorphic core complex formation, from the onset of delamination (a), development of low angle faults (b), isostatic uplift of the Lower Plate (c), to exposure of the mylonitic front in the final stages of continental extension (d). Note the dragging of mylonitised granite along the youngest detachment fault, during its emplacement into the Lower Plate (Lister and Davis 1989). Figure 1.2………………………………………………………………………….p. 26

Diagram illustrating the proposed behaviour of the Moho during continental extension and associated isostatic uplift of the Lower Plate in metamorphic core complexes. The Moho is considered to be initially horizontal (a), and may either become uplifted by mantle flow (b), or remain flat with ductile flow being accommodated in the lower crust (c) (Block and Royden 1990). Figure 1.3………………………………………………………………………….p. 28

Five schematic diagrams of crustal extension associated with detachment faulting, including: a single through-going detachment (A), a detachment with ramps and flats (B), stretching of the lithosphere by pure shear without influencing the adjacent rift basins (C), stretching of the lithosphere by pure shear directly beneath the detachment (D), and a combination of ramps and flats with ductile stretching beneath the detachment (E) (Lister et al. 1991). Figure 1.4………………………………………………………………………….p. 33

Schematic diagram illustrating the configuration of the Median Tectonic Zone and the Eastern and Western Provinces of New Zealand, prior to the late-Cenozoic displacement associated with the onset of the Alpine Fault plate boundary (Bradshaw 1993). Figure 1.5………………………………………………………………………….p. 35

Geological map outlining the basement terranes of the NW Nelson-Westland and correlatives in the Fiordland area, with granitoid outcrop locations and emplacement ages in the Buller and Takaka Terranes. The location of Western Province rocks is shown in the inset, both before and after the onset of the Alpine Fault (Muir et al. 1994). Figure 1.6………………………………………………………………………….p. 37

Geological map of the Paparoa Metamorphic Core Complex, with the Ohika and Pike Detachment Faults located to the north and south of the Paparoa Range, respectively. Upper Plate (cover plate) and Lower Plate (basement plate) rocks are shown, along with the bedding and foliation measurements of various units (Tulloch and Kimbrough 1989). Inset of study location from Spell et al. (2000).

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Figure 1.7………………………………………………………………………….p. 39

Schematic crosssection showing the asymmetric passive margins of the Tasman Sea Basin, which developed in the final stages of rifting after the development of the Paparoa Metamorphic Core Complex (Tulloch and Kimbrough 1989). Figure 2.1………………………………………………………………………….p. 48

Schematic crosssection showing the basic structure of the Paparoa Metamorphic Core Complex, with the Ohika and Pike Detachment Faults located on the north-eastern and south-western sides of the isostatically uplifting Lower Plate. The deformed Charleston Metamorphic Group (CMG) rocks and granitoids are shown in the Lower Plate, with the Pororari Group sediments adjacent to both detachment faults in the Upper Plate (Spell et al. 2000). Figure 2.2……………………………………………………………………..pp. 53-54

Geological map of the Lower Buller Gorge area on the West Coast of New Zealand (Nathan 1978). Figure 2.3………………………………………………………………………….p. 56

Geological map of the Pike Stream area in the Paparoa Range, West Coast, New Zealand (Hume 1977). Figure 2.4……………………………………………………………………..pp. 57-58

Geological map of the Foulwind-Charleston area on the West Coast of New Zealand (Nathan 1975). Figure 2.5………………………………………………………………………….p. 61

Geological map of own observations in the Lower Buller Gorge area, with granitoid rocks being classified under the adopted basement deformation scheme (from I to III). Locations of samples for geochemical analyses are shown (RS-35 to RS-40), with foliation and lineation measurements shown for deformed granitoids. The inferred location of the Ohika Detachment Fault is shown. Scale = 1:41,500. Figure 2.6………………………………………………………………………….p. 62

Geological map of own observations in the Pike Stream area of the Paparoa Range, with granitoid rocks being classified under the adopted basement deformation scheme (from II to IV). Scale = 1:33,300.

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Figure 2.7………………………………………………………………………….p. 64

Geological map of own observations along the coastal section north of Fox River Mouth (inset), with granitoid rocks being classified under the adopted basement deformation scheme (from II to V). Location of samples for geochemical analyses are shown (PCC06-01 and PCC06-02), with foliation and lineation measurements shown for deformed granitoids. The inferred location of the Pike Detachment Fault is shown. Scale of inset = 1:33,300; scale of coastline = 1:50,000. Figure 2.8…………………………………………………pp. 68-69, 71-72, 74, 76-77.

Thin section images of hand specimens RS-35 to RS-40 from the Lower Buller Gorge area and PCC06-01 from the coastal section north of Fox River Mouth, at 2.5x and 10x magnification. Scale bars = 1 cm; CPL. Figure 3.1a…………………………………………………………………………p. 86

Cathodoluminescence images of individual zircon grains from sample RNZ119. Grains from left to right: B2-1, B2-2, B2-3, B2-4, B2-5 and B2-6. Scale bars: 50 µm in length. Geochemical sampling locations shown as circles for oxygen (solid lines), hafnium (short dashes) and U-Pb + trace elements (long dashes). Figure 3.1b………………………………………………………………………...p. 86

Cathodoluminescence images of individual zircon grains from sample UC08368. Grains from left to right: 68-4, 68-5, 68-1, 68-3 and 68-2. Scale bars: 50 µm in length. Geochemical sampling locations shown as circles for oxygen (solid lines), hafnium (short dashes) and U-Pb + trace elements (long dashes). Figure 3.1c…………………………………………………………………………p. 87

Cathodoluminescence images of individual zircon grains from sample PCC06-01. Grains from left to right: S-1, S-2, S-3, S-4, S-8, S-7, S-6, S-5, S-9, S-10 and S-11. Scale bars: 50 µm in length. Geochemical sampling locations shown as circles for oxygen (solid lines), hafnium (short dashes) and U-Pb + trace elements (long dashes). Figure 3.1d……………………………………………………………………...…p. 87

Cathodoluminescence images of individual zircon grains from sample UC05592. Grains from left to right: 92-1, 92-3, 92-2 and 92-4. Scale bars: 50 µm in length. Geochemical sampling locations shown as circles for oxygen (solid lines), hafnium (short dashes) and U-Pb + trace elements (long dashes). Figure 3.2a…………………………………………………………………………p. 94

Cathodoluminescence images of zircons from sample RNZ119 from the Paparoa Metamorphic Core Complex. Various morphologic features are shown, as follows: ‘C’ for corrosion; ‘CF’ for concentric fracturing and ‘SP’ for damage associated with sample processing.

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Figure 3.2b……………………………………………………………………..…p. 95

Cathodoluminescence images of zircons from sample UC08368 from the Paparoa Metamorphic Core Complex. Morphologic features are shown as follows: ‘C’ for corrosion and ‘GT’ for ‘ghost’ textures. Figure 3.2c…………………………………………………………………………p. 97

Cathodoluminescence images of zircons from sample PCC06-01 from the Paparoa Metamorphic Core Complex. Morphologic feature shown: ‘M’ for metamictization. Figure 3.2d……………………………………………………………………...…p. 99

Cathodoluminescence images of zircons from sample UC05592 from the Paparoa Metamorphic Core Complex. Various morphologic features are shown, as follows: ‘C’ for corrosion; ‘CF’ for concentric fracturing; ‘GT’ for ‘ghost’ textures; ‘M’ for metamictization and ‘SP’ for damage associated with sample processing. Figure 4.1………………………………………………………………………...p. 106

Bivariate plots of SiO2 vs. TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O and P2O5 for samples UC05592, PCC06-01, UC08368, RS-35 to RS-40 and RNZ119, from the Paparoa Metamorphic Core Complex. Figure 4.2………………………………………………………………………...p. 117

Plot of 87Sr/86Sr vs. 87Rb/86Sr of a mylonite (sample PCC06-02) from White Horse Creek at the southern end of the Paparoa Metamorphic Core Complex. A maximum age of ductile deformation is calculated as 116.2 ± 5.9 Ma (Ring et al. 2006). Figure 4.3a……………………………………………………………………..…p. 126

Plot of SiO2 vs. REE’s of rocks adjacent to the Pike and Ohika Detachment Faults of

the Paparoa Metamorphic Core Complex (Tulloch and Christie 2000). Figure 4.3b…………………………………………………………………….…p. 126

Plot of chondrite-normalised REE’s of unaltered granitoids of the Paparoa Metamorphic Core Complex (Tulloch and Christie 2000). Figure 4.4a………………………………………………………………………..p. 135

REE chondrite-normalised diagram showing values of zircon xenocrystic cores from sample UC05592.

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Figure 4.4b……………………………………………………………………….p. 136

REE chondrite-normalised diagram showing values of zircon rims from sample UC05592. Figure 4.4c………………………………………………………………………..p. 136

REE chondrite-normalised diagram showing average REE values of xenocrystic cores and rims of zircons from sample UC05592. Figure 4.4d……………………………………………………………………….p. 138

REE chondrite-normalised diagram showing values of zircon xenocrystic cores from sample UC08368. Figure 4.4e………………………………………………………………………..p. 138

REE chondrite-normalised diagram showing values of zircon rims from sample UC08368. Figure 4.4f………………………………………………………………………..p. 139

REE chondrite-normalised diagram showing average REE values of xenocrystic cores and rims of zircons from sample UC08368. Figure 4.4g……………………………………………………………………….p. 139

REE chondrite-normalised diagram showing values of zircon xenocrystic cores from sample RNZ119. Figure 4.4h……………………………………………………………………….p. 140

REE chondrite-normalised diagram showing values of zircon rims from sample RNZ119. Figure 4.4i………………………………………………………………………..p. 142

REE chondrite-normalised diagram showing average REE values of xenocrystic cores and rims of zircons from sample RNZ119. Figure 4.4j………………………………………………………………………..p. 142

REE chondrite-normalised diagram showing values of zircon xenocrystic cores from sample PCC06-01.

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Figure 4.4k……………………………………………………………………….p. 143

REE chondrite-normalised diagram showing values of zircon rims from sample PCC06-01. Figure 4.4l……………………………………………………………………..…p. 143

REE chondrite-normalised diagram showing average REE values of xenocrystic cores and rims of zircons from sample PCC06-01. Figure 4.5a………………………………………………………………………..p. 148

Probability density distribution-histogram plot of all data for sample UC05592. Figure 4.5b……………………………………………………………………….p. 148

Probability density distribution-histogram plot of inherited c. 300 Ma zircons for sample UC05592. Figure 4.5c………………………………………………………………………..p. 149

Tera-Wasserburg concordia diagram of sample UC05592. Data-point error ellipses are 2σ. Figure 4.6a………………………………………………………………………..p. 150

Probability density distribution-histogram plot of all data for sample UC08368. Figure 4.6b……………………………………………………………………….p. 150

Probability density distribution-histogram plot of inherited c. 300 to c. 400 Ma zircons for sample UC08368. Figure 4.6c………………………………………………………………………..p. 151

Tera-Wasserburg concordia diagram of sample UC08368. Data-point error ellipses are 2σ. Figure 4.7a………………………………………………………………………..p. 152

Probability density distribution-histogram plot of all data for sample PCC06-01. Figure 4.7b……………………………………………………………………….p. 152

Probability density distribution-histogram plot of data <700 Ma for sample PCC06-01.

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Figure 4.7c………………………………………………………………………..p. 153

Tera-Wasserburg concordia diagram of all data from sample PCC06-01. Data-point error ellipses are 1σ. Figure 4.8a………………………………………………………………………..p. 154

Probability density distribution-histogram plot of all SHRIMP data for sample RNZ119 (Muir et al. 1994). Figure 4.8b……………………………………………………………………….p. 155

Probability density distribution-histogram plot of SHRIMP data excluding inherited zircons for sample RNZ119 (Muir et al. 1994). Figure 4.9a………………………………………………………………………..p. 156

Probability density distribution-histogram plot of all data for sample RNZ119. Figure 4.9b……………………………………………………………………….p. 156

Probability density distribution-histogram plot excluding inherited zircons for sample RNZ119. Figure 4.9c………………………………………………………………………..p. 157 Tera-Wasserburg concordia diagram of all data from sample PCC06-01. Data-point error ellipses are 2σ. Figure 4.10……………………………………………………………………….p. 159

δ18O ‰ [SMOW] vs. U-Pb age (Ma) for sample RNZ119. Arrows link cores and rims from individual zircon grains. Average δ18O mantle value of 5.3 ± 0.3 ‰ shown as the stippled area. Figure 4.11……………………………………………………………………….p. 159

δ18O ‰ [SMOW] (zircon cores only) vs. δ18O ‰ [SMOW] (zircon rims only) for sample RNZ119. Average δ18O mantle value of 5.3 ± 0.3 ‰ shown as the stippled area. Figure 4.12a………………………………………………………………………p. 161

Histogram of zircon xenocrystic cores and rims from sample UC08368.

8

Figure 4.12b……………………………………………………………………..p. 161

Histogram of zircon xenocrystic cores and rims from sample RNZ119. Figure 4.12c………………………………………………………………………p. 161

Histogram of zircon xenocrystic cores and rims from sample PCC06-01. Figure 4.12d……………………………………………………………………...p. 161

Histogram of zircon xenocrystic cores and rims from sample UC05592. Figure 4.13……………………………………………………………………….p. 161

Ti-in-zircon T (˚C) vs. U-Pb age (Ma) for sample RNZ119. Arrows link cores and rims from individual zircon grains. Figure 4.14……………………………………………………………………….p. 162

Ti-in-zircon T (˚C) vs. δ18O ‰ [SMOW] for sample RNZ119. Arrows link cores and rims from individual zircon grains. Average δ18O mantle value of 5.3 ± 0.3 ‰ shown as the stippled area. Figure 4.15……………………………………………………………………….p. 162

Ti-in-zircon T (˚C) vs. εHf (zircon) for sample RNZ119. Arrows link cores and rims from individual zircon grains. Figure 4.16……………………………………………………………………….p. 164

εHf vs. U-Pb age (Ma) for sample RNZ119. Arrows link cores and rims from individual zircon grains. Figure 4.17……………………………………………………………………….p. 164

εHf (zircon cores only) vs. εHf (zircon rims only) for sample RNZ119. Buckland Granite TDM calculated using the U-Pb zircon crystallisation age of 110.3 ± 0.9 Ma. Figure 4.18……………………………………………………………………….p. 165

εHf (zircon) vs. δ18O ‰ [SMOW] for sample RNZ119. Arrows link cores and rims from individual zircon grains. Average δ18O mantle value of 5.3 ± 0.3 ‰ shown as the stippled area.

9

Figure 4.19……………………………………………………………………….p. 166

Zr/Hf vs. U-Pb age (Ma) for sample RNZ119. Arrows link cores and rims from individual zircon grains. Figure 4.20……………………………………………………………………….p. 166

εHf (zircon) vs. Zr/Hf for sample RNZ119. Arrows link cores and rims from individual zircon grains. Figure 4.21……………………………………………………………………….p. 168

Ti-in-zircon T (˚C) vs. Zr/Hf for sample RNZ119. Arrows link cores and rims from individual zircon grains. Figure 4.22……………………………………………………………………….p. 168

εHf (zircon) vs. Th/U for sample RNZ119. Arrows link cores and rims from individual zircon grains. Figure 4.23……………………………………………………………………….p. 171

Probability-density distribution-histogram plots of the Karamea Batholith and Victoria Paragneiss (Greenland Group metasediments), Muir et al. (1996). Figure 5.1……………………………………………………………………….p. 182

Schematic crosssection of the Paparoa Metamorphic Core Complex, with the Ohika and Pike Detachment Faults to the north and south, respectively. The proposed 102 and 110 Ma magmatic pulses are illustrated, with the 110 Ma pulse being dragged to the south in response to movement along the Pike Detachment Fault from c. 116 Ma (Ring et al. 2006).

10

LIST OF TABLES Table 2.1…………………………………………………………………………...p. 66

Percentage estimates of thin sections for hand specimens RS-35 to RS-40 from the Lower Buller Gorge area and hand specimen PCC06-01 from the coastal section north of Fox River Mouth. Table 3.1…………………………………………………………………………...p. 80

Comparisons between the use of individual minerals and whole rocks in geochemical investigations (Amelin et al. 2000). Table 3.2a……………………………………………………………………….…p. 88

U, Th and Th/U ratios of zircon grains with 208Pb corrected 206Pb/238U ages from sample RNZ119. Alternating font style (bold vs. regular) show analyses common to individual zircon grains. Analyses in italics are sample spots which were not able to be distinguished as cores or rims. Table 3.2b………………………………………………………………………….p. 89

U, Th and Th/U ratios of zircon grains with 208Pb corrected 206Pb/238U ages from sample UC08368. Alternating font style (bold vs. regular) show analyses common to individual zircon grains. Analyses in italics (“?”) are sample spots which were not able to be distinguished as cores or rims. Table 3.2c………………………………………………………………………….p. 90

U, Th and Th/U ratios of zircon grains with 208Pb corrected 206Pb/238U ages from sample PCC06-01. Alternating font style (bold vs. regular) show analyses common to individual zircon grains. Analyses in italics (“?”) are sample spots which were not able to be distinguished as cores or rims. Table 3.2d………………………………………………………………………….p. 91

U, Th and Th/U ratios of zircon grains with 208Pb corrected 206Pb/238U ages from sample UC05592. Alternating font style (bold vs. regular) show analyses common to individual zircon grains. Analyses in italics (“?”) are sample spots which were not able to be distinguished as cores or rims. Table 3.3…………………………………………………………………………..p. 92

Long/short axis and aspect ratios for zircons from the four samples from the Paparoa Metamorphic Core Complex.

11

Table 3.4…………………………………………………………………………..p. 93

Aspect ratios from samples RNZ119, UC08368, UC05592 and PCC06-01 from the Paparoa Metamorphic Core Complex. The range of zircon grain lengths (in µm), minimum, maximum and mean aspect ratios are shown. Table 4.1a………………………………………………………………………...p. 104

XRF results of major elements from samples UC05592, PCC06-01, UC08368, and RS-35 to RS-40. Table 4.1b………………………………………………………………………..p. 104

XRF results of trace elements from samples UC05592, PCC06-01, UC08368, and RS-35 to RS-40. Table 4.1c………………………………………………………………………..p. 105

XRF results of major elements within the sample RNZ119, carried out at the University of Canterbury, Christchurch, New Zealand (Muir et al. 1997). Table 4.1d……………………………………………………………………..…p. 105

XRF results of trace elements within the sample RNZ119, carried out at the University of Canterbury, Christchurch, New Zealand (Muir et al. 1997). Table 4.2………………………………………………………………………….p. 109

Oxygen isotope data for samples RNZ119 (n2056), PCC06-01 (n2064), UC05592 (n2065) and UC08368 (n2068) from the Paparoa Metamorphic Core Complex. Table 4.3………………………………………………………………………….p. 109

Average δ18O data with +/- per mil (2dp) for RNZ119 (n2056), PCC06-01 (n2064), UC05592 (n2065) and UC08368 (n2068. Table 4.4………………………………………………………………………….p. 112

Titanium contents of RNZ119 (Buck), PCC06-01 (S), UC08368 (68) and UC05592 (92). Alternating font style (bold vs. regular) shows analyses common to individual zircon grains. Analyses in italics are samples spots which are not common to individual grains. ‘?’ = analyses which may not be distinguished between cores or rims; bdl = below detection level.

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Table 4.5a………………………………………………………………………...p. 113

Zircon saturation thermometry of the Buckland Granite (RNZ119) using titanium content, based on the method outlined in Watson et al. (2006). Alternating font style (bold vs. regular) shows analyses common to individual zircon grains. Analyses in italics are samples spots which were not able to be distinguished as cores or rims, and are not included in the δT (°C) calculations. Table 4.5b………………………………………………………………………...p. 115

Zircon saturation thermometry of zircon xenocrystic cores from sample RNZ119 based on the method outlined in Watson et al. (2006). Table 4.5c………………………………………………………………………..p. 115

Zircon saturation thermometry of zircon rims from sample RNZ119 based on the method outlined in Watson et al. (2006). Table 4.6a……………………………………………………………………..…p. 121

Hafnium data for samples RNZ119, UC08368, UC5592 and PCC06-01. Alternating font style (bold vs. regular) shows analyses common to individual zircon grains. Analyses in italics are samples spots which were not able to be distinguished as cores or rims, and are not included in the Hf calculations. Table 4.6b………………………………………………………………………...p. 121

Hafnium data for standards used for the samples UC08368, UC5592 and PCC06-01. Table 4.7……………………………………………………………………….…p. 122

Combined LA-ICP-MS data for sample RNZ119, with calculated values for εHf and TDM for all analyses. The calculation of εHf follows the method used by Nebel et al. (2007), with U-Pb ages sourced from individual zircon grains. Averages and standard deviations are shown for εHf, εHf max (uncorrected), εHf min (uncorrected), HfDM at T and calculated TDM. Table 4.8a………………………………………………………………………p. 123

Combined LA-ICP-MS data for sample RNZ119, with calculated values for εHf and TDM for zircon xenocrystic cores only. The calculation of εHf follows the method used by Nebel et al. (2007), with U-Pb ages sourced from individual zircon grains. Averages and standard deviations are shown for εHf, εHf max (uncorrected), εHf min (uncorrected), HfDM at T and calculated TDM.

13

Table 4.8b………………………………………………………………………...p. 123

Combined LA-ICP-MS data (Hf, U and Pb) for sample RNZ119, with calculated values for εHf and TDM for zircon rims only. The calculation of εHf follows the method used by Nebel et al. (2007), with U-Pb ages sourced from individual zircon grains. Averages and standard deviations are shown for εHf, εHf max (uncorrected), εHf min (uncorrected), HfDM at T and calculated TDM. Table 4.9a………………………………………………………………………..p. 128

LA-ICP-MS results for sample UC05592, where bdl: below detection level. Table 4.9b………………………………………………………………….pp. 128-129

LA-ICP-MS results for sample UC08368, where bdl: below detection level. Table 4.9c…………………………………………………………………..pp. 131-132

LA-ICP-MS results for sample RNZ119, where bdl: below detection level. Table 4.9d………………………………………………………………….pp. 133-134

LA-ICP-MS results for sample PCC06-01, where bdl: below detection level. Table 4.10……………………………………………………………………..….p. 154

SHRIMP data for the Buckland Granite (sample RNZ119) by Muir et al. (1994). Table 5.1a………………………………………………………………………...p. 176

Combined geochemical analysis details for sample RNZ119 (Buckland Granite), with zircon grain number, location of analysis (core vs. rim) and U-Pb zircon ages. Table 5.1b………………………………………………………………………...p. 177

Combined geochemical analysis details for sample UC08368, with zircon grain number, location of analysis (core vs. rim) and U-Pb zircon ages. Table 5.1c………………………………………………………………………..p. 178

Combined geochemical analysis details for sample PCC06-01, with zircon grain number, location of analysis (core vs. rim) and U-Pb zircon ages.

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Table 5.1d……………………………………………………………………..…p. 179

Combined geochemical analysis details for sample UC05592, with zircon grain number, location of analysis (core vs. rim) and U-Pb zircon ages. Table 5.2………………………………………………………………………….p. 181

102 vs. 110 Ma zircons from the Buckland Granite (sample RNZ119), with averages for U, Th/U and Ti-in-zircon T (˚C). Table 5.3………………………………………………………………………….p. 183

U, Th, Th/U and U-Pb data for samples ‘Stitts Tuff I’ and Stitts Tuff II’ (Muir et al. 1997).

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ACKNOWLEDGEMENTS I would like to thank my supervisors Dr. Uwe Ring and Associate Professor John Bradshaw for their enthusiasm and assistance in this project. Many thanks to Dr. Robert Bolhar for his help in geochemical work while in Australia and advice in the department. Thanks also to Dr. J. Michael Palin from the University of Otago for his involvement in the LA-ICP-MS work in Canberra. Thanks to Mr. Rob Spiers and Mr. Stephen Brown for their technical support, and to Dr. David Shelley for his help with optical mineralogy. Financial support from the Mason Trust Fund. To Julia Hofmann, who travelled from Germany to join me in the adventure of field work on the West Coast of New Zealand. And finally, to my family and friends, who provided me with ongoing support and encouragement.

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ABSTRACT

Cretaceous continental extension was accommodated by the development of the

Paparoa Metamorphic Core Complex, resulting in the separation of New Zealand

from Gondwana. High grade (Lower Plate) and low grade (Upper Plate) rocks are

separated by the Ohika and Pike Detachment Faults. The two detachment faults have

distinctly different histories, with greater exhumation along the Pike Detachment

Fault. The onset of crustal extension is proposed to have commenced along the Pike

Detachment Fault at 116.2 ± 5.9 Ma (Rb/Sr dating). Both geochemical and

geochronological approaches are adopted for this thesis, through the in situ analysis of

oxygen and hafnium isotope ratios, trace metals and U-Pb content. Chemical changes

are tracked during the petrogenesis of the Buckland Granite, with mafic replenishment

observed in the later stages of crystallisation. Crystallisation temperatures of the

Buckland Granite are calculated using zircon saturation thermometry, with an average

Ti-in-zircon temperature of 697°C (upper-amphibolite facies). Inherited zircons in

Lower Plate rocks show distinct age peaks at c. 1000, 600 and 300 Ma, illustrating the

incorporation of heterogeneous local crust (Greenland Group and Karamea Batholith).

Model ages (TDM) are calculated for inherited zircons of the Lower Plate rocks, which

record the time at which magma bodies (zircon host rocks) were extracted from the

mantle. Maximum and minimum model ages for the Buckland Granite average at

3410 Ma and 2969 Ma, with the maximum TDM value of 3410 Ma coinciding with the

proposed major crustal formation event of the Gondwana supercontinent at c. 3.4-3.5

Ga. Two distinct U-Pb zircon age peaks are observed in the Buckland Granite at 102.4

± 0.7 and 110.3 ± 0.9 Ma. The 110.3 ± 0.9 Ma age is interpreted as the crystallisation

age of the pluton, while the 102.4 ± 0.7 is proposed to represent a younger thermal

(magmatic?) event associated with the 101-102 Ma Stitts Tuff.

17

CHAPTER 1: INTRODUCTION

The development of metamorphic core complexes takes place in areas in which regional

extension follows a period of contraction, which resulted in the over-thickening of the

continental crust. Relationships between Upper and Lower Plate rocks, and the role of

detachment faulting still continue to bemuse geologists, with opposing views on the

growth of ductile shear zones, and conflict over the role of plutonism and igneous

underplating on isostatic uplift.

1.1. Metamorphic core complexes Early investigations into the development of mountain systems saw the description of

central ‘axial core zones’, within which ductile flow was believed to take place (Eskola

1949). Observations of the associated metamorphism of crustal rocks resulted in such

structures being classified as gneissic domes. The juxtaposition of deformed material

against relatively undeformed cover rocks was noted in the gneissic dome structures, and

referred to as ‘infrastructure’ and ‘suprastructure’ domains, respectively. These domains

were seen to be sharply separated by regional-scale ‘decollements’, along which steep

metamorphic gradients and structural discontinuities exist (Eskola 1949, Misch 1960).

The tectonic relationship between these distinctly different domains was first addressed

by Price and Mountjoy (1970), who illustrated that the uplift of ‘infrastructure’ rocks and

coeval lateral movement were intimately linked. The gravitationally-driven movement

away from the uplifting block was also proposed to be the driving force of adjacent

foreland fold and thrust belt development.

18

Armstrong (1972) extended upon this tectonic interpretation by stating that

‘decollements’ were low-angle ‘denudation’ faults, along which significant displacement

took place at the regional-scale.

Many of the early investigations focussed on either the presence of contractional or

extensional structures only, and failed to note that the two opposing tectonic regimes are

both of importance (Coney and Harms 1984). The post-plate tectonics interpretation of

metamorphic terranes along the major mountain ranges of the North American Cordillera,

along with revised interpretations of gneissic dome structures, led to the eventual

development of the metamorphic core complex theory (Coney 1980). Nomenclature

earlier suggested for gneissic dome structures were replaced by more appropriate terms,

with ‘infrastructure’, ‘suprastructure’ and ‘decollements’ renamed as Lower Plate, Upper

Plate and detachment faults, respectively. The Lower Plate is positioned below

detachment faults in the foot wall, while the Upper Plate rests above detachment faults in

the hanging wall (Coney 1980). This revised nomenclature has since become widely used

in the scientific community.

The Lower Plate consists of ductilely deformed rocks of the lower crust, which have

experienced upper-amphibolite to middle-greenschist facies metamorphism under the

extensional regime. Lower Plate rocks are often referred to as the ‘basement rocks’ of

metamorphic core complexes, and commonly contain plutonic rocks emplaced into mid-

crustal levels. Upper Plate rocks, on the other hand, have not experienced ductile

deformation during extension, and often consist of both sedimentary and volcanic units of

the upper crust. The Upper Plate forms the cover sequence of metamorphic core

19

complexes, with multiple stages of brittle deformation recorded in the variety of faults

present within the upper crust (Lister and Davis 1989). It is important to note that

variations in both the composition and age of units in both the Lower and Upper Plates of

metamorphic core complexes may be considered to be irrelevant with respect to the

nature of extension (Coney 1980).

The development of metamorphic core complexes commonly post-dates a period of

contraction, in response to the over-thickening of the continental crust. Contraction often

encourages plutonism at depth, resulting in further strain being added to the crust.

Granitoid plutons emplaced within Lower Plate domains during extension are frequently

found to have a garnet-bearing, two-mica type mineralogy, which develops in response to

the melting of over-thickened crustal material at depth. The link between granitoid

mineralogy and the structural nature of metamorphic core complexes has been suggested

to provide insight into both the petrogenetic history and tectonic evolution of such

systems (Coney 1980).

Thermal relaxation of the continental crust follows after the cessation of contraction, in

response to the ending of convergent tectonics (Brun et al. 1994). The pre-thickening and

subsequent thermal relaxation of the crust leads to the development of detachment faults,

to accommodate regional-scale extension. The juxtaposition of highly deformed rocks

against relatively undeformed rocks occurs along these detachment faults, allowing the

Lower and Upper Plate rocks to rest in tectonic contact. The dragging of the Lower Plate

from beneath the Upper Plate results in the alteration and brittle deformation of rocks

20

adjacent to the active detachment faults. Chloritisation, cataclasis and brecciation of

Lower Plate rocks develops in response to their exhumation into shallower crustal depths,

resulting in a quartz-sericite assemblage (Coney 1980, Davis et al. 1983, Davis et al.

1986, Tulloch and Palmer 1990).

The evolution of faulting within metamorphic core complexes has been addressed by

many authors, resulting in a variety of models being suggested to address the mechanism

of crustal extension.

The development of metamorphic core complexes was initially proposed to take place

under simple shear regimes, in which a single detachment fault would exist through time,

in an ‘evolving shear zone’ situation. In this model, the detachment fault was considered

to represent the brittle-ductile transition zone (continental stress guide), with brittle

deformation being restricted to the Upper Plate, and ductile deformation within the

Lower Plate only (Wernicke 1981a, Wernicke 1981b, Miller et al. 1983, Wernicke 1983,

Wernicke 1985).

This model does not account for igneous activity commonly associated with

metamorphic core complexes, and does not acknowledge magmatism as a direct

consequence of continental crustal extension. This model has been discarded on several

grounds, with evidence against the suggestion that brittle and ductile deformation must

take place at the same period within the Upper and Lower Plates, respectively. In this

model, mylonitisation of rocks may only occur during the faulting of a single detachment,

which does not account for the observed overprinting of both brittle and ductile structures

in metamorphic core complexes (Lister and Davis 1989).

21

Two other models of interest both acknowledge the growth of several large-scale shear

zones within the crust through time, with the development of these detachment faults

through the exploitation of older normal faults associated with the earlier stages of

extension. The overprinting of ductile and brittle structures commonly observed is thus

accounted for, through the multiple generations of detachment faults within individual

metamorphic core complexes. The difference between the two models is based on the

angle at which the older normal faults lie.

The development of detachment faults from initially low-angle normal faults was

proposed by Davis and co workers (1980), to account for the low-angle listric nature of

ductile shear zones. This model is not widely accepted, as several low-angle normal

faults at close intervals would be required to promote extension, which is considered to

be mechanically unlikely (Lister and Davis 1989).

The alternate model suggests the development of detachment faults from high-angle

normal faults, at angles between 30-60˚. Block rotation accommodates extension through

the rotation of high-angle faults, resulting in the development of low-angle listric

detachment faults (Buck 1988, Wernicke and Axen 1988, Lister and Davis 1989, Brun et

al. 1994, Wjins et al. 2005). This model is considered to be structurally feasible, with

detachment faults representing the upper equivalents of ductile shear zones at depth

(Davis et al. 1983, Lister et al. 1984, Lister and Snoke 1984, Reynolds and Spencer 1985,

Wernicke 1985, Davis et al. 1986, Lister and Davis 1989).

The development of multiple detachment faults within an individual metamorphic core

complex takes place, through the splaying of shear zones from a primary master fault at

22

depth, beneath an extending upper plate. Primary master faults are thought to develop

along the brittle-ductile transition (continental stress guide), as regional-scale structures

able to govern structural responses within both the brittle and ductile crust (Lister and

Davis 1989). Splaying of detachment faults from the primary master fault may occur in

response to stresses associated with the isostatic uplift of the Lower Plate, in attempt to

maintain ductile flow. Continued isostatic uplift through continued extension of the

continental crust, results in the formation of multiple generations of detachment faults

through time. As the isostatic uplift continues, previously active detachment faults

become inactive, to be replaced by new detachment faults. The cessation in activity of

detachment faults results from their warping during isostatic uplift, causing the

detachment faults to lie at different orientations to the regional extensional direction.

New detachment faults therefore splay off the primary master fault, with low-angle listric

geometries to accommodate extension. Inactive shear zones remain preserved within the

Lower Plate of metamorphic core complexes, with characteristic bow-like geometries.

The bowing up of old shear zones into shallower crustal levels experiencing active

detachment faulting results in the exposure of the mylonitic front, through the horizontal

back-rotation of the mylonitic rocks (Figure 1.1; Lister and Davis 1989, Reynolds and

Lister 1990).

Heterogeneous strain softening of the continental crust is required for the

accommodation of regional-scale strain under an extensional regime. The development of

shear zones through strain softening allows the ductile flow of material at depth, which is

not possible through bulk deformation of the surrounding country rock. Strain softening

23

Figure 1.1: Model of metamorphic core complex formation, from the onset of delamination (a), development of low angle faults (b), isostatic uplift of the Lower Plate (c), to exposure of the mylonitic front in the final stages of continental extension (d). Note the dragging of mylonitised granite along the youngest detachment fault, during its emplacement into the Lower Plate (Lister and Davis 1989).

24

may take place through the decrease of stress at a constant strain rate, through the

increase in strain rate at constant stress, or through the involvement of both mechanisms

(White et al. 1980). Cataclasis along detachment faults promotes continued strain

softening, which may also be accelerated through the incorporation of water sourced

from crystallising granitoid plutons (if present) in the Lower Plate (Lister and Davis

1989).

Minerals within mylonitised polymineralic rocks respond differently to deformation,

with harder minerals being able to withstand strain and remain as porphyroclasts, and the

softer minerals of the matrix undergoing recrystallisation. Mylonitic rocks developed

through the extreme ductile deformation of quartzo-feldspathic rocks will therefore

consist of feldspar porphyroclasts, with a quartz-dominated recrystallised matrix (White

et al. 1980). The plastic deformation of plagioclase feldspar and alkali feldspar may occur

in mylonites which have experienced amphibolite to granulite facies metamorphism

(Goode 1978). Granitoid plutons commonly emplaced in the Lower Plate are subject to

such deformation, with the degree of metamorphism directly correlating to the position

from the shear zone.

The role of ductile flow in metamorphic core complexes has been the subject of debate,

with varying views on the depth at which this style of deformation occurs. Ductile flow

within the mid- and lower-crust has been suggested as the main mechanism encouraging

regional-scale extension (Block and Royden 1990, Bertotti et al. 2000), rather than

simply flow strictly within the asthenosphere (Spencer 1984).

25

In the model with flow occurring strictly within the asthenosphere, detachment faulting

removes hanging wall material through extension and/or erosion, therefore reducing the

vertical load of the continental crust. The subsequent development of a horizontal

pressure gradient at depth encourages the lateral flow of material towards the central

region of a developing metamorphic core complex. Ductile flow of material within the

asthenosphere is proposed to be the main mechanism driving the extension, through the

isostatic uplift of the Moho at the regional-scale (Figure 1.2; Spencer 1984, Block and

Royden 1990). Mid-crustal rocks undergoing extension, however, do not support this

theory, as many may become ductile at temperatures as low as 280˚C, through the

deformation and recrystallisation of quartz.

Ductile flow of material within the mid-crust is considered to be a more likely

mechanism responsible for regional-scale extension. Isostatic uplift and subsequent

thinning of the continental crust is shown to take place without the isostatic uplift of the

Moho, through the upwelling of the weakened lower crust only (Figure 1.2; Block and

Royden 1990).

The symmetry or asymmetry of metamorphic core complexes at the regional scale

depends on the style of detachment faulting, as shown by numerical modelling (Lister et

al. 1991). The rheology of the crust strongly influences the structural geometry of

metamorphic core complexes, with symmetric extension commonly taking place in hot,

weakened crust, and asymmetric extension in cold, stiff crust (Ranalli and Murphy 1987,

Bertotti et al. 2000, Wjins et al. 2005). The extension of the crust may take place through

a

26

Figure 1.2: Diagram illustrating the proposed behaviour of the Moho during continental extension and associated isostatic uplift of the Lower Plate in metamorphic core complexes. The Moho is considered to be initially horizontal (a), and may either become uplifted by mantle flow (b), or remain flat with ductile flow being accommodated in the lower crust (c) (Block and Royden 1990).

27

variety of mechanisms, with extension being accommodated by large detachment faults

(Figure 1.3). Symmetric metamorphic core complexes involve two detachment faults,

positioned on either side of the uplifting Lower Plate. Asymmetric metamorphic core

complexes, on the other hand, involve only one detachment fault, along which the Lower

Plate is dragged from underneath the Upper Plate.

Lower Plate rocks of symmetric metamorphic core complexes may be intruded by

vapour-present (‘wet’) plutonic rocks, in which monazite and zircon are common. The

degree of dissolution of monazite is increased by the presence of vapour, with plutons

characteristically showing elevated levels of light-Rare Earth Elements (LREE’s) and

heavy-Rare Earth Elements (HREE’s). Asymmetric metamorphic core complexes

commonly involve the intrusion of vapour-undersaturated (‘dry’) plutonic rocks, with

monazite, zircon ± garnet present in the residual melt. LREE’s and HREE’s are

anomalously low in concentration, while positive Eu spikes are commonly observed

(Watt and Harley 1993).

The chemistry of vapour-present (‘wet’) plutonic rocks is commonly observed in S-type

granitoid rocks, emplaced into previously over-thickened continental crust during

extension at the regional-scale (Watt and Harley 1993).

The development of metamorphic core complexes, regardless of mechanism, requires

the isostatic uplift of (at least) the continental crust at the regional-scale. Lower Plate

rocks dragged out from beneath the Upper Plate become uplifted to elevations above sea

level, to produce the characteristic dome-like profile (Coney 1980, Lister and Davis

1989, Waight et al. 1998b). Metamorphic unroofing of the Lower Plate encourages the

erosion and transportation of clastic material along active detachment faults, to become

28

Figure 1.3: Five schematic diagrams of crustal extension associated with detachment faulting, including: a single through-going detachment (A), a detachment with ramps and flats (B), stretching of the lithosphere by pure shear without influencing the adjacent rift basins (C), stretching of the lithosphere by pure shear directly beneath the detachment (D), and a combination of ramps and flats with ductile stretching beneath the detachment (E) (Lister et al. 1991).

29

deposited in non-marine basins along outer margins of metamorphic core complexes

(Coney 1980, Brun et al. 1994, Bertotti et al. 2000).

This phase of extension is characterised by the ductile flow of material towards the

central region, with subsidence of the hanging walls and uplift of the footwalls along

detachment faults (Bertotti et al. 2000). The mylonitisation of rocks continues under the

ductile regime, and persists until accommodation of the extension is reached through the

development of detachment faults (White et al. 1980). Gravitational instability associated

with the domal geometry leads to horizontal spreading and crustal extension, further

accelerating isostatic uplift.

Mylonitic rocks exposed at the surface of ancient metamorphic core complexes

represent older phases of ductile deformation, which become uplifted by younger phases

of detachment faulting, which develop as individual splays to accommodate extension.

The mechanism responsible for isostatic uplift is subject to debate, with opposing views

on the role of plutonism and igneous underplating in metamorphic core complexes.

Plutonism and igneous underplating of the Lower Plate may continue for an extended

period of time, until the reservoir becomes expended. The role of rheology of the

continental crust, however, is proposed to be the dominant driving force of isostatic

uplift, with plutonism and igneous underplating considered to be of secondary importance

(Brun et al. 1994, Wjins et al. 2005).

Variations in rheology results in the division of the continental crust into two layers

through mechanical stratification during thermal relaxation (Ranalli and Murphy 1987,

Brun et al. 1994, Wjins et al. 2005), with the upper crust consisting of strong, cold rocks

30

(Lister and Davis 1989), and the lower crust of hot, weak rocks (Gans 1987, Block and

Royden 1990). Clearly rheological layering influences responses of the crust to tectonic

activity, with brittle failure in the upper crust, and ductile deformation taking place in the

lower crust.

Both vertical and lateral contrasts in the rheology of the crust may exist in response to

tectonic denudation of the continental crust, with lateral variations considered to be

subordinate to the vertical rheology during continental extension. Vertical contrasts in

rheology are considered the controlling mechanism during metamorphic core complex

formation, supported by both analogue and numerical modelling of crustal extension

(Brun et al. 1994, Wjins et al. 2005). Brun and co workers (1994) propose that plutonism

and/or igneous underplating of the Lower Plate is required for the weakening of the lower

crust, to allow metamorphic core complex formation. Numerical modelling by Wjins and

co workers (2005), however, illustrates that this is not necessary, provided there is a

uniformly weak zone within the lower crust through basic contrasts in rheology.

The eventual death of metamorphic core complexes follows the loss of isostatic uplift,

with the shift from dominantly ductile to the more brittle style of deformation and

associated subsidence in both foot- and hanging walls of the detachment faults (Lister et

al. 1991, Bertotti et al. 2000). This shift is facilitated by the cessation of ductile flow at

depth, through rheological changes in the crust. Large strength ratios responsible for the

exhumation of the weak lower crust change to small strength ratios, resulting in the

strengthening of the lower crust. The isostatic uplift of the Lower Plate may no longer

31

continue, as the lower crust becomes more rigid in shallower crustal levels and inhibits

ductile flow (Wjins et al. 2005).

Normal faulting of the upper crust develops perpendicular to the older ductile structures,

parallel to stretching lineation directions (Davis et al. 1980). Brittle faults become

superimposed upon the older ductile structures, along structurally weakened zones

(Coney 1980, Lister et al. 1991). Rotation of crustal blocks takes place during the tilting

of the high-angle normal faults to accommodate continued extension in the upper crust,

resulting in the development of younger low-angle listric detachment faults (Lister and

Davis 1989, Rosenbaum et al. 2005).

Rigid block faulting and rotation of crustal blocks in metamorphic core complexes is

believed to represent the final stages of extension prior to the eventual separation of

continental crust, and formation of oceanic crust (Coney 1980, Reynolds and Spencer

1985, Lister and Davis 1989, Rosenbaum et al. 2005). The axis of brittle extension

commonly develops within basement rocks of the Lower Plate, where isostatic uplift was

at its greatest during the earlier ductile phase of extension (Falvey and Mutter 1981).

Brittle faulting associated with newly forming oceanic crust often show direct

geometrical relationships with the older ductile features, with spreading ridges commonly

forming normal to the older extensional direction (Coney 1980, Lister et al. 1991).

Continental margins previously joined prior to extension commonly display differences,

in both structural features and uplift-subsidence patterns on either side of the spreading

ridge (Lister et al. 1991).

32

1.2.Regional geology

A terrane approach has been adopted in New Zealand for the correlation of rock units

older than 100 million years in age. The South Island consists of around ten major

terranes which have been grouped into the Median Tectonic Zone, the Eastern Province,

and the Western Province (Figure 1.4; Bradshaw 1989, Bradshaw 1993). It is important

to note that each terrane is an incomplete section, which must be taken into consideration

when piecing together the geological record. Rocks of the South Island have also

experienced 480 km of lateral displacement along the Miocene to Recent Alpine Fault,

with an associated overprinting of previous tectonic structures by the Cenozoic

movement (Landis and Coombs 1967).

The Median Tectonic Zone consists of a complex elongate section of plutonic, volcanic

and volcanically-derived sediments formed along magmatic arc(s) during the

Carboniferous to Cretaceous period (Bradshaw 1993, Kimbrough et al. 1993, Muir et al.

1998, Mortimer et al. 1999). Plutonism occurred in a convergent tectonic regime, due to

convergence of the Phoenix and Pacific Plates and over-thickening of the continental

crust during this period

The Eastern Province is dominated by Permian to early Cretaceous volcanics and

accretionary material, deposited along the eastern margin of Gondwana. Material from

both the Median Tectonic Zone and the Eastern Province prior to the Early Cretaceous

developed under a convergent tectonic regime.

33

Figure 1.4: Schematic diagram illustrating the configuration of the Median Tectonic Zone and the Eastern and Western Provinces of New Zealand, prior to the late-Cenozoic displacement associated with the onset of the Alpine Fault plate boundary (Bradshaw 1993).

34

The oblique collision of the Phoenix and Pacific Plates with the eastern margin of

Gondwana during the Early Cretaceous resulted in an almost immediate switch from

contractional to extensional tectonics (Laird 1981, Bradshaw 1989), and initiated further

emplacement of granitoid rocks into the volcanic arc. Such Cretaceous granitoids are

found in both the Median Tectonic Zone, as discussed above, and the Western Province.

The Western Province has been divided into the Takaka and Buller Terranes, which are

separated by the Anatoki Fault (Bradshaw 1989).

The Takaka Terrane consists of clastic material, limestones, quartzites and volcanics

ranging from Cambrian to Devonian in age. Mafic rocks and granitoids were then

emplaced into the Terrane during the Devonian and Cretaceous, respectively.

The majority of the Buller Terrane consists of the Greenland Group which was

deposited during the Cambrian to Ordovician period, and makes up the ‘basement’ of this

terrane. The Greenland Group is a quartz-rich metamorphosed flysch unit, non-calcareous

and unfossiliferous in nature, with a minimum Rb-Sr age of 495 ± 11 Ma (Adams 1975,

Hume 1977, Kimbrough and Tulloch 1989).

The Buller Terrane consists largely of Paleozoic metasediments peppered with various

granitoid rocks intruded during the Devonian, Carboniferous and Early Cretaceous

(Figure 1.5; Muir et al. 1994, Muir et al. 1995, Muir et al. 1998, Waight et al. 1998b).

These granitoid rocks have been divided into three suites by Tulloch (1988) on the basis

of variations in both age and geochemistry. The Karamea Suite is dominated by S-type

granitoids ranging in age from the Devonian to the Carboniferous. The Separation Point

Suite is Cretaceous in age, and consists of I-type granitoids, including the Separation

Point Granite. The final group is referred to as the Rahu Suite, which contains transitional

35

Figure 1.5: Geological map outlining the basement terranes of the NW Nelson-Westland and correlatives in the Fiordland area, with granitoid outcrop locations and emplacement ages in the Buller and Takaka Terranes. The location of Western Province rocks is shown in the inset, both before and after the onset of the Alpine Fault (Muir et al. 1994).

36

I/S-type granitoids, alkali-lamprophyre dikes and the Charleston Metamorphic Group

(Nathan 1975, Tulloch and Kimbrough 1989). The Hohonu Suite to the south is the

correlative of the Rahu Suite due to similarities in age and petrology (Tulloch 1988,

Waight et al. 1997, Waight et al. 1998b).

The majority of granitoids within the Rahu Suite are biotite granites and granodiorites,

with some muscovite-bearing granitoids. The Buckland Granite is included in the Rahu

Suite, and is of particular interest to this thesis in regards to the development of the

Paparoa Metamorphic Core Complex in the Western Province.

The Western Province was then disturbed by the emplacement of Cretaceous calc-

alkaline Rahu Suite granitoids (Buckland Granite), and experienced extreme deformation

during the development of the Paparoa Metamorphic Core Complex under an extensional

regime (Tulloch and Kimbrough 1989, Waight et al. 1998b).

The Paparoa Metamorphic Core Complex is bound by two master detachments, the

Ohika and Pike Detachment Faults to the north and the south of the Paparoa Range,

respectively (Figure 1.6). The Ohika Detachment Fault dips to the NNE with a ‘top-to-

the-NNE’ shear sense, while the Pike Detachment dips to the SSW with a ‘top-to-the-

SSW’ sense of shear (Tulloch and Kimbrough 1989).

Sedimentary basins developed rapidly in response to the initiation of crustal extension,

in both grabens and half-grabens. The non-marine Mid Cretaceous Pororari Group was

deposited in basins NE and SW of the Paparoa Range (Figure 1.6), with beds generally

dipping to the SW in the north-eastern basins, and NW in the south-eastern basins. The

37

Figure 1.6: Geological map of the Paparoa Metamorphic Core Complex, with the Ohika and Pike Detachment Faults located to the north and south of the Paparoa Range, respectively. Upper Plate (cover plate) and Lower Plate (basement plate) rocks are shown, along with the bedding and foliation measurements of various units (Tulloch and Kimbrough 1989). Inset of study location from Spell et al. (2000).

38

Pororari Group comprises of a basal non-marine mudstone, the 101-102 Ma vitric Stitts

Tuff, and sandstone overlain by the coarse Hawks Crag Breccia (Tulloch and Kimbrough

1989, Muir et al. 1997). The Stitts Tuff was initially proposed to be the volcanic

equivalent of the nearby Berlins Porphyry pluton (Bowen 1964). Dates for both the Stitts

Tuff and the Berlins Porphyry were later found to be c. 101-102 and 110 Ma,

respectively, which disproves an igneous link between the two groups (Muir et al. 1997).

Rotation of the Hawks Crag Breccia accommodated continued extension of the brittle

upper crust during the final stages of detachment faulting. Movement along the Pike

Detachment Fault is suggested to have continued for some time after the Ohika

Detachment Fault, with exhumation of the Lower Plate until c. 98 Ma along the Ohika

Detachment Fault, and c. 85 Ma along the Pike Detachment Fault (Tulloch and Palmer

1990). The c. 30 Ma of activity along the detachment faults is considered to be too long,

with a life span of c. 10 Ma thought to be more likely (e.g. Ring et al. 2007).

NNE-SSW extension led to the eventual separation of the New Zealand section from

Gondwana, through the development of the NNW-SSE Tasman Basin spreading margin

by c. 84 Ma (Figure 1.7; Laird 1981, Grindley and Davey 1982, Bradshaw 1989, Tulloch

and Kimbrough 1989, Muir et al. 1992, Laird 1993, Waight et al. 1998b, Spell et al.

2000, Laird and Bradshaw 2004). Emplacement of the alkaline A-type French Creek

Granite, and the associated alkaline-basalt Hohonu dike swarm at 81.7 ± 1.8 Ma in a

dominantly WNW orientation, corresponds with the onset of sea floor spreading (Tulloch

and Kimbrough 1989, Tulloch et al. 1994, Waight 1994, Waight et al. 1997, Waight et al.

1998a).

39

Figure 1.7: Schematic crosssection showing the asymmetric passive margins of the Tasman Sea Basin, which developed in the final stages of rifting after the development of the Paparoa Metamorphic Core Complex (Tulloch and Kimbrough, 1989).

Marine transgressions during the Mid Tertiary resulted in the blanketing of the

Charleston Metamorphic Group and Pororari Group rocks (Tulloch and Kimbrough

1989). Subsequent erosion, transportation and deposition of material over time resulted in

the present day exposure level of the West Coast.

1.3. Thesis objectives

Observations in the geology of South Island terranes over the years have provided a

multitude of information related to the evolution of New Zealand, and its relationship

with the Gondwana supercontinent during the Paleozoic and Mesozoic eras. The model

of the Paparoa Metamorphic Core Complex proposed by Tulloch and Kimbrough in 1989

is relatively simplistic, and has led to several questions. This thesis brings together

observations and interpretations made by previous workers on the initiation, development

and cessation of the Paparoa Metamorphic Core Complex, and attempts to extend upon

40

such data with structural, geochemical and geochronological approaches. Objectives for

this particular thesis are outlined below.

1. Carry out a detailed review of the behaviour of metamorphic core complexes in

general, and compare with current knowledge of the Paparoa Metamorphic Core

Complex.

2. Determine whether the degree of deformation in Lower Plate rocks corresponds to

their position in respect to the adjacent detachment faults.

3. Interpret deformational structures and textures of a variety of rocks from the

complex, at both the hand specimen and thin section scale.

4. Carry out detailed geochemical and geochronological investigations using both

the in situ sampling of individual zircon minerals, and the ‘whole rock’ approach

of hand specimens from the complex. Sample various elements and trace metals

to achieve greater understanding of the petrogenesis and metamorphism of rocks

from the complex, to build on and compare with geochemical findings by

previous studies.

5. Calculate the crystallisation temperatures reached during the period of crustal

extension, and attempt to constrain the timing of detachment faulting through the

dating of the mylonitisation at either ends of the complex. Directly correlate the

timing (U-Pb dating) of crystallisation with calculated temperatures (based on Ti

content), by the detailed in situ sampling of individual zircon grains.

6. Attempt to better constrain the timing of crystallisation of the Buckland Granite.

41

7. Establish the role of detachment faulting in the emplacement of the Buckland

Granite. Determine whether the pluton is likely to have been syn-tectonic or late

tectonic in nature, and whether the Pike Detachment Fault became more dominant

than the Ohika Detachment Fault during extension. If so, what effect did this

dominance in movement to the south have on the pluton?

8. Address the issue of inheritance through detailed geochemical and

geochronological investigations into the provenance of a variety of rocks from the

complex.

9. Calculate the timing at which the zircon-host rocks were initially extracted from

the mantle, and link to major crustal formation events in the early stages of

Earth’s history.

10. List potential questions for future research in the Paparoa Metamorphic Core

Complex.

1.4. Location of study

The Paparoa Metamorphic Core Complex is located in the Western Province on the

West Coast of the South Island, New Zealand. It is important to note that the West Coast

of the South Island is dominated by dense rainforest, with limited exposure. Outcrops are

considerably difficult to find, with steep terrain further limiting access to certain areas.

Areas of interest include the Lower Buller Gorge near Westport, a section along the

coastline south of Charleston, and the Pike Stream area in the Paparoa Range near

Greymouth. Outcrops in the Lower Buller Gorge area are accessible along both rail and

road side cuttings, and within the major river systems. The coastline south of Charleston

42

provides a more accessible sequence of outcrop, from Morrissey Creek to the Fox River

Mouth. The Pike Stream, on the other hand, has limited outcrops found within nearby

river systems.

1.5. Methodology

In order to gain a greater understanding of the mechanisms involved in continental

extensional tectonics, and the nature of the Paparoa Metamorphic Core Complex, a

variety of approaches must be taken.

1.5.1. Field work

Four weeks of intensive field work was undertaken in the Lower Buller Gorge area, the

coastal section from Charleston to the Fox River Mouth, and the Pike Stream area in the

Paparoa Range. Foliations and stretching lineations were recorded during the structural

mapping of these areas, in an attempt to show any regional pattern of deformation

associated with the development of the core complex.

The degree of deformation of the core complex-related rocks was addressed through the

development of a ‘basement deformation classification’ system, based on field

observations and thin section analysis. Five levels of deformation were qualitatively

established, as follows:

(I) Undeformed granitoid,

(II) Deformed granitoid,

(III) Orthogneiss (strongly deformed granitoid),

(IV) Mylonite, and

43

(V) Ultramylonite

This classification system was used in attempt to illustrate the relationship between the

degree of deformation of the Paparoa Metamorphic Core Complex rocks and their

location with respect to the adjacent detachment faults.

The collection of oriented hand specimens provided material for geochemical analysis,

and allowed the degree of ductile deformation to be observed at the thin section scale.

1.5.2. Geochemical work: CL imaging, SIMS and LA-ICP-MS

Four samples from the Paparoa Metamorphic Core Complex were both sourced from

PETLAB (UC08368, UC05592 and RNZ119) and collected during field work (PCC06-

01), for both the in situ sampling of individual zircon minerals, and whole rock

geochemical sampling. These two approaches were adopted in attempt to further

constrain the chemical makeup and history of the core complex, and to determine

whether the sampling methods carried out in this thesis (documented core vs. rim

analyses of individual zircon grains) provide more detailed findings than previous

investigations.

PETLAB samples were sourced from the following locations:

1. UC08368 – Lat/Long 41.83683˚S; 171.66402˚E (NZGD49),

2. UC05592 – Lat/Long 41.84829˚S; 171.70589˚E (NZGD49), and

3. RNZ119 – Lat/Long 41.84909˚S; 171.7158˚E (NZGD49).

44

Zircon separation was carried out through the crushing of hand specimens with a

hydraulic press, and the sieving of material for particles between 50 and 350µm. Zircon

grains were then extracted using standard heavy liquid and magnetic techniques, using a

Frantz separator. Individual zircons were handpicked under a binocular microscope,

mounted in epoxy resin and then polished to expose the interior sections of grains.

Zircons from each of the four samples were then digitally imaged using

cathodoluminescence (CL) by Dr. Robert Bolhar at the University of Otago. CL imaging

was undertaken prior to sampling for labelling and publication purposes, as analysis by in

situ Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and

Multi-Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-IPC-MS) is

destructive. The nature of individual zircons, such as growth patterns, lattice defects, and

overgrowth structures, are made visible through CL imaging, enabling accurate in situ

geochemical sampling of specific sites of interest within individual grains.

Oxygen isotopes were analysed for each of the four samples by Dr. Robert Bolhar and

Dr. Martin Whitehouse in the NordSIMS facility at the Swedish Museum of Natural

History using the Secondary Ion Mass Spectrometry (SIMS) technique, following the

procedures outlined in Nemchin and co workers (2006a), Nemchin and co workers

(2006b) and Schuhmacher and co workers (1994). Each mount was polished to expose

any xenocrystic cores present, and coated in approximately 30nm of gold in preparation

for the analysis.

45

LA-ICP-MS was carried out at the Australian National University in Canberra, and at

the University of Melbourne, Australia. Procedures followed those described by Ballard

and co workers (2001) and Woodhead and co workers (2004).

The four samples from the Paparoa Metamorphic Core Complex were analysed using

the pulsed Lambda Physik LPX 120I UV ArF excimer laser in the Research School of

Earth Sciences at the Australian National University. The excimer laser operates at 70mJ

and 5Hz to allow spot sizes of 24µm or 32µm, with penetration depths of around 20µm.

Mixed He-Ar gas transported ablated material from the sample cell and flow

homogeniser to the Aglient 7500 Quadrupole ICP-MS.

The in situ analysis of hafnium was carried out in the School of Earth Sciences at the

University of Melbourne, using the Nu Plasma MC-ICP-MS coupled to a 193-nm ArF

excimer laser, with a spot size of 30-40µm operating at 60mJ and total intensity of 1-2V.

Each analysis commenced with 20 seconds of background data collection, followed by

40 seconds of laser ablation, in which trace metals and elemental ratios were recorded. A

period of 30 to 60 seconds without ablation between each sample, often referred to as a

‘gas blank’ period, allowed the stabilisation of background levels to ensure consistency

within the data.

Data reduction involved the removal of outliers and background levels collected during

spot analysis. Exclusion of the first 10 scans in each spot analysis reduced the effect of

instrumental mass bias, by removing unsteady ion counts. Correction for the inter-

element fractionation of Pb, Th, and U involved the averaging of known values from

46

standard zircons Temora and 91500, and the silicate glass NIST SRM 610 (Wiedenbeck

et al. 1995, Pearce et al. 1997, Black et al. 2003).

1.5.3. Geochemical work: whole rock analysis (XRF)

Ten samples from the Paparoa Metamorphic Core Complex were used for whole rock

analysis, including the four samples also used for trace metal/U-Pb dating, as outlined

above. The remaining six samples were collected along an East-West profile through the

Lower Buller Gorge area.

Whole rock samples were crushed using the hydraulic press, until broken into smaller

clasts, and then run through the Ring Mill until the samples were of a floury consistency.

Samples were then analysed using the X-Ray Fluorescence (XRF) process after Norrish

and Hutton (1969), modified by Harvey and co workers (1973) and Norrish and Chappell

(1977) for major and trace elements, respectively.

The classification of rocks from the Paparoa Metamorphic Core Complex allows direct

comparisons to be made between samples, and provides information on the composition

of these samples for additional geochemical work.

47

CHAPTER 2: STRUCTURAL GEOLOGY

The Paparoa Metamorphic Core Complex displays many characteristic features of

Cordilleran-type metamorphic core complexes, with similarities in both the structural

geology and mineralogy of associated rocks.

2.1. Previous work

The observed juxtaposition of relatively undeformed rocks above high grade

metamorphic rocks in the Paparoa Range has recently been interpreted as an ancient

metamorphic core complex, associated with the separation of the New Zealand

section from Gondwana during the Cretaceous (Tulloch and Kimbrough 1989).

Development of the symmetric Paparoa Metamorphic Core Complex accommodated

the regional-scale extension through the development of two detachment faults north

and south of the Paparoa Range, referred to as the Ohika and Pike Detachment Faults,

respectively (Figure 2.1). The Ohika Detachment Fault dips to the NNE, with a ‘top-

to-the-NNE’ sense of shear, while the Pike Detachment dips to the SSW with ‘top-to-

the-SSW’ shear sense. Regional extension of the continental crust took place in a

NNE-SSW orientation, in both the Upper and Lower Plates on either side of the

detachment faults (Kimbrough and Tulloch 1989). Previous authors investigating

structural relationships in the Paparoa Range noted the opposing shear sense patterns,

which remains consistent with the metamorphic core complex model (Laird 1967,

Hume 1977). The opposing sense of shear is recorded within the S-C fabrics of

mylonitic rocks, and illustrates the bivergent style of continental extension during this

period (Tulloch and Kimbrough 1989, Spell et al. 2000).

48

Figure 2.1: Schematic crosssection showing the basic structure of the Paparoa Metamorphic Core Complex, with the Ohika and Pike Detachment Faults located on the north-eastern and south-western sides of the isostatically uplifting Lower Plate. The deformed Charleston Metamorphic Group (CMG) rocks and granitoids are shown in the Lower Plate, with the Pororari Group sediments adjacent to both detachment faults in the Upper Plate (Spell et al. 2000).

The onset of ductile deformation in the Paparoa Metamorphic Core Complex is

recorded at 114 ± 18 Ma in the Charleston Metamorphic Group orthogneiss

(Kimbrough and Tulloch 1989), and at 116.2 ± 5.9 Ma in the ultramylonite at

Morrisey Creek (Ring et al. 2006). Ductile deformation took place under upper-

amphibolite facies metamorphic conditions, as recorded in deformed feldspars within

mylonitic rocks of the Lower Plate (Hutton et al. 1990, White 1994).

Continental extension was facilitated by this ductile regime, through the mechanism

of ductile flow at depth, resulting in the isostatic uplift of the Lower Plate through

detachment faulting. Isostatic uplift may have been further accelerated by the coeval

emplacement of the Buckland Granite.

The Buckland Granite was emplaced into the Lower Plate at 109.6 ± 1.7 Ma (Muir et

al. 1992), covering an area of approximately 180 km². However, there is only one

49

single U-Pb age known for the Buckland Granite and the question is whether this age

is actually representative for the intrusion age of the huge pluton.

The biotite-muscovite + garnet granite displays the characteristic mineralogy

expected for plutons emplaced under an extensional regime, with approximately 1km²

of the pluton being garnet bearing (Coney 1980, Graham and White 1990).

Sedimentary material became incorporated into the Buckland Granite magma during

its emplacement, resulting in the inheritance of pre-existing zircons from a variety of

sources. The presence of sedimentary input into the Buckland Granite allows the

pluton to be classified as a S-type granite, with low temperature S-type melts

generally becoming more felsic as they evolve through the crust (Chappell and White

1974, Chappell and White 1992, Clemens 2003, Chappell et al. 2004).

The last stages of ductile shearing within the Paparoa Range are recorded within the

Buckland Granite, which suffered from varying degrees of ductile deformation during

emplacement (Muir et al. 1992, Waight et al. 1998b, Spell et al. 2000). Emplacement

of the Buckland Granite post-dated the timing of upper amphibolite-facies

metamorphism of the Charleston Metamorphic Group orthogneisses, as reflected in

the metamorphic mineralogy of the pluton (Tulloch and Kimbrough 1989, White

1994).

The presence of an undeformed lamprophyre dike in the Lower Buller Gorge area

provides relative timing on the cessation of ductile deformation at that location, with

an age of c. 80 to 86 Ma (Tulloch and Kimbrough 1989).

The onset of upper crustal extension is believed to post-date the ductile regime by 5

to 10 Ma (Tulloch and Kimbrough 1989, Laird 1993, Muir et al. 1997). However,

50

strain-compatibility arguments make this inference questionable because upper and

lower crust have to undergo the same amount of extension at the same time.

Extension within the upper crust was accommodated by the development of high-

angle normal faults striking approximately 120˚, which tilted to form low-angle

detachment faults through the rotation of crustal blocks of the Upper Plate (Tulloch

and Kimbrough 1989).

The detachment faults at either end of the complex became footwalls to the newly

forming, asymmetric non-marine basins. The coeval metamorphic unroofing of the

Lower Plate fed clastic material into the basins adjacent to the Ohika Detachment

Fault from c. 102 to 90 Ma, to form the Hawks Crag Breccia of the Pororari Group

(Tulloch and Palmer 1990, Muir et al. 1992, Spell et al. 2000). Clast compositions

record changes in source material during metamorphic unroofing, with an upward

change from clasts of the Greenland Group metasediments, to the incorporation of

granitoid clasts (Nathan 1978, Tulloch and Palmer 1990). The timing of deposition of

the Hawks Crag Breccia in basins adjacent to the Pike Detachment Fault, however, is

not well constrained due to a lack of a dateable horizon. It is possible that the breccia

older in the southern basins may be c. 15 Ma older than the northern deposits.

Extension is suggested to have initially been controlled by exhumation along the

Ohika Detachment Fault, and is proposed to predate the Pike Detachment Fault

(Tulloch and Palmer 1990, Spell et al. 2000). Deposition of c. 102 Ma Hawks Crag

Breccia in the northern basins is proposed to have taken place during this early stage

of metamorphic unroofing, along the transport direction of the Ohika Detachment

Fault (Tulloch and Kimbrough 1989). Exhumation along the Ohika Detachment Fault

encouraged the emplacement of the c. 109 Ma Buckland Granite into the thinning

51

Lower Plate adjacent to the shear zone. However, the timing of deposition of the

Hawks Crag Breccia and of emplacement of the Buckland Granite does not fit, as the

Hawks Crag Breccia (containing Buckland Granite clasts) is considerably younger

than the pluton. If the Ohika detachment moved already by 109 Ma and the Buckland

granite was synkinematic to the detachment, then the critical question would

be as to where the upper crustal extension has been accommodated between 109

and 102 Ma.

Thermal relaxation of the crust followed, causing the locking up of the Ohika

Detachment Fault and subsequent shift in exhumation along the newly forming Pike

Detachment Fault (Spell et al. 2000).

Movement along the Pike Detachment Fault resulted in the dragging of the

syntectonic Buckland Granite to the south at depth during emplacement, accelerating

the cooling and metamorphic unroofing of the pluton (Hutton et al. 1990, Waight et

al. 1998b, Spell et al. 2000). The dominance of Buckland Granite clasts within the

Hawks Crag Breccia in the southern basins can be accounted for by the dragging of

the pluton and the then adjacent Hawks Crag Breccia deposits to the south (Graham

and White 1990, Spell et al. 2000).

Beds within the Hawks Crag Breccia, deposited parallel to the adjacent detachment

faults during exhumation, became rotated towards the centre of the metamorphic core

complex under the brittle regime – resulting in the tilting of beds towards the uplifted

Paparoa Range (Tulloch and Kimbrough 1989, Spell et al. 2000). The tilting of the

Hawks Crag Breccia continued after deposition ceased, through continued

exhumation along the detachment faults, very soon after the breccia started to be

deposited at c. 101-102 Ma (Tulloch and Kimbrough 1989).

52

The second stage of extension involved the complete separation of the New Zealand

section from the Gondwana supercontinent, through the development of oceanic crust

by c. 84 Ma (Grindley and Davey 1982). The NNW-SSE Tasman Sea spreading ridge

developed perpendicular to the previous extension direction responsible for the

Paparoa Metamorphic Core Complex, as expected in Cordilleran-style metamorphic

core complexes (Figure 1.7; Coney 1980, Tulloch and Kimbrough 1989). The

injection of dikes parallel to the newly forming spreading ridge reflects the change of

extension direction, with dikes exploiting previously weakened upper crustal rocks

(Tulloch and Kimbrough 1989, Waight 1994, Waight 1995).

The Pike Detachment Fault has been proposed to have continued to be active until c.

85 Ma, based on K-Ar sericite dating, with the Pike Detachment Fault suggested to

play a great role in the eventual separation of the New Zealand section from

Gondwana through the development of the Tasman Sea Basin (Tulloch and Palmer

1990). The timing of extension, however, is more likely to have ceased earlier, at c.

90 Ma, with ~10-15 Ma of relative quiescence before the complete separation of New

Zealand from Gondwana at c. 84 Ma.

2.2. The Paparoa Metamorphic Core Complex

The structure of the Paparoa Metamorphic Core Complex has been investigated by

previous authors, with the locations of the Ohika and Pike Detachment Faults being

inferred by the tectonic contact seen between Lower and Upper Plate rocks. It is

important to note the difficultly in pinpointing the location of detachment faults due to

the limited access to outcrops on the West Coast.

The Ohika Detachment Fault outcrops along State Highway 6 in the Lower Buller

Gorge, and separates the Greenland Group, Pororari Group and Berlins Porphyry of

53

the Upper Plate from the Buckland Granite, Railway Quartz Diorite and the Windy

Point Granite of the Lower Plate (Figure 2.2; Nathan 1978). No exposure of the Ohika

Detachment Fault has been observed, but has been inferred on the basis of brecciated

N

Scale = 1:63,360

Figure 2.2: Geological map of the Lower Buller Gorge area on the West Coast of New Zealand (Nathan 1978).

54

Figure 2.2 (continued): Legend for the geological map of the Lower Buller Gorge area on the West Coast of New Zealand (Nathan 1978).

55

mylonitic rocks found within the upper reaches of the Orowaiti Rivers (Tulloch and

Kimbrough 1989).

The Pike Detachment Fault is located at the southern end of the Paparoa Range in

the Pike Stream area (Figure 2.3), extending to the highly deformed rocks exposed

along the coastline north of the Fox River Mouth (Figure 2.4; Nathan 1975). Upper

Plate rocks in the Pike Stream area are extensively silicified at the top of the range,

with large quartz veins running through the Greenland Group metasediments. The

silicification of material is thought to represent alteration of rocks adjacent to the

shear zone, which may be used to infer the ESE trending contact southwest of Mount

Bovis (Tulloch and Kimbrough 1989). The nearby White Knight Stream contains a

quartz-rich contact (Nathan 1978), which may represent the continuation of the Pike

Detachment Fault to the south. The detachment fault is proposed to have been located

a few hundred metres above the current topography in the Pike Stream area, with only

the deep crustal equivalent rocks being exposed at present (Tulloch and Kimbrough

1989).

Several models have been proposed for mechanisms responsible for the development

of metamorphic core complexes, based on the sense of shear along detachment faults.

The uniform-simple model proposed by Wernicke (1985) and Lister and Davis (1989)

requires a uniform sense of shear across the whole metamorphic core complex.

The Paparoa Metamorphic Core Complex does not strictly match this model, with

opposing sense of shear at either end of the Paparoa Range, associated with

movement along the Ohika and Pike Detachment Faults. Opposing shear sense is

recorded in the Upper Plate cover rocks, with the tilted Pororari Group units dipping

towards the central region of the Paparoa Range (Tulloch and Kimbrough 1989).

56

Figure 2.3: Geological map of the Pike Stream area in the Paparoa Range, West Coast, New Zealand (Hume 1977).

57

N

Scale = 1:63,360

Figure 2.4: Geological map of the Foulwind-Charleston area on the West Coast of New Zealand (Nathan 1975).

58

Figure 2.4 (continued): Legend for the geological map of the Foulwind-Charleston area on the West Coast of New Zealand (Nathan 1975).

59

Exhumation and slip rates have been calculated for the Paparoa Metamorphic Core

Complex detachment faults, based on a variety of techniques. Exhumation rates

reflect a vertical component of movement, while slip rates refer to movement along a

fault plane at any particular angle.

Tulloch and Palmer (1990) calculated a minimum slip rate of 1.1-1.4 mm/yr from

depths of c. 25km, from Buckland and Blackwater Granite clasts in the Hawks Crag

Breccia deposits adjacent to the Ohika Detachment Fault in the Lower Buller Gorge.

The slip rate along the Pike Detachment Fault was calculated to be 0.6-1.0mm/yr – a

slower rate than that proposed by Tulloch and Palmer (1990). Pressure and

temperature estimates for the Charleston Metamorphic Group rocks adjacent to the

Pike Detachment Fault were calculated at 4 ±1 kbar and 600 ± 50˚C, respectively

(White 1994). Based on the P-T estimates, a geothermal gradient of c. 50-90˚C/km for

the Paparoa Metamorphic Core Complex was calculated by White (1994).

The proposed geothermal gradients of White (1994) were used in a later paper by

Spell and co workers (2000), for the calculation of cooling rates for exhumed rocks

within the Paparoa Metamorphic Core Complex. A cooling rate of c. 110˚C/Ma was

proposed for the c. 330˚C cooling of rocks from c. 500-170˚C, with 4-7km of

exhumation along the Pike Detachment Fault at a slip rate of 1-2mm/yr.

2.3. Own observations

Additional observations were made during the period of field work, with the

adoption of a basement deformation classification scheme in areas adjacent to both

the Ohika and Pike Detachment Faults to determine whether the degree of ductile

deformation increases towards the shear zones as expected.

60

The deformation of Lower Plate rocks in the Lower Buller Gorge was found to

follow a progressive increase towards the inferred Ohika Detachment Fault, ranging

from outcropping undeformed granitoid rocks through to orthogneiss (Figure 2.5). No

mylonite was directly observed during the period of field work, but mylonitic float

sourced from the upper reaches of the Little Cascade Creek has been noted by earlier

investigations (Ledlie 1989-1990). The increased deformation of rocks adjacent to the

Ohika Detachment Fault follows the expected pattern of metamorphic gradients.

However, deformation directly below the Ohika Detachment Fault is entirely

cataclastic in nature, and resulted in a several hundred metres thick cataclastically

deformed zone in the footwall of the Ohika Detachment Fault.

Contact metamorphism of the Greenland Group metasediments resulted in the

development of andalusite, probably in response to the emplacement of the Buckland

Granite into the Lower Plate. The andalusite present within the cataclastic Upper Plate

rocks is immediately juxtaposed against the Buckland Granite, suggesting the offset

along the Ohika Detachment Fault was relatively limited.

Rocks of the Pike Stream area experienced a high degree of exhumation, with

mylonitic deformation observed. Deformation becomes cataclastic towards the

inferred Pike Detachment Fault, with the overall nature of deformation being

heterogeneous (Figure 2.6). The heterogeneous pattern of deformation is also

observed in the coastal section north of the Fox River Mouth. Lower Plate rocks

exposed at the coastline show a progressive decrease in metamorphic grade towards

the shear zone, with undeformed granitoid rocks and cataclastic rocks outcropping

near the inferred detachment fault. Ductile deformation becomes increasingly stronger

61

Figure 2.5: Geological map of own observations in the Lower Buller Gorge area, with granitoid rocks being classified under the adopted basement deformation scheme (from I to III). Locations of samples for geochemical analyses are shown (RS-35 to RS-40), with foliation and lineation measurements shown for deformed granitoids. The inferred location of the Ohika Detachment Fault is shown. Scale = 1:41,500

62

Figure 2.6: Geological map of own observations in the Pike Stream area of the Paparoa Range, with granitoid rocks being classified under the adopted basement deformation scheme (from II to IV). Scale = 1:33,300.

63

to the north, through deformed granitoid rocks to the mylonites and ultramylonites of

Morrisey Creek (Figure 2.7).

Greater exhumation took place along the Pike Detachment Fault compared to the

Ohika Detachment Fault, with greater offset occurring along the Pike Detachment

Fault to allow the exhumation of the mylonites and ultramylonites of Morrisey Creek.

The ductile deformation of basement rocks along the coastline section does not follow

the expected pattern, with respect to the inferred location of the Pike Detachment

Fault. Deformation is observed to be heterogeneous, where mylonitisation took place

at depth and became cataclastically overprinted towards the detachment. This rapid

transition from ultramylonites (c. 350-500˚C) to cataclasite (c. 150-250˚C) is exposed

over a distance of c. 500m along the coastline. Using this 350-500˚C temperature

difference and the geothermal gradient of 50-90˚C/km (White 1994), an estimated 4-

10km of material is cut out over the distance of c. 500m. Mylonites of the Paparoa

Metamorphic Core Complex record NNE stretching lineations plunging to both the

north and south of the Paparoa Range, which is regionally consistent with the

transport directions of Upper Plate rocks during exhumation (Tulloch and Kimbrough

1989). The majority of deformed rocks at Morrisey Creek have NE trending

stretching lineations, with NE-SW oriented foliations (Figure 2.7). Stretching

lineations in the Lower Plate rocks are oriented sub-parallel to the earlier subduction

zone along the eastern margin of Gondwana, and perpendicular to the later Tasman

Sea spreading centre (Tulloch and Kimbrough 1989, Tulloch and Palmer 1990).

2.4 Hand specimens

Hand specimens were collected from the Lower Buller Gorge area (samples RS-35

to RS-40) and the coastline north of Fox River Mouth (sample PCC06-01).

64

Figure 2.7: Geological map of own observations along the coastal section north of Fox River Mouth (inset), with granitoid rocks being classified under the adopted basement deformation scheme (from II to V). Location of samples for geochemical analyses are shown (PCC06-01 and PCC06-02), with foliation and lineation measurements shown for deformed granitoids. The inferred location of the Pike Detachment Fault is shown. Scale of inset = 1:33,300; scale of coastline = 1:50,000.

65

River Mouth (Figures 2.5 and 2.7). The Lower Plate rocks collected within the Lower

Buller Gorge (hand specimens RS-35 to RS-40) all display variation in the degree of

deformation, from relatively undeformed to slightly deformed in nature. The mylonite

hand specimen (PCC06-01), sourced north of the Fox River Mouth, displays a distinct

mylonitic fabric, with smeared feldspar phenocrysts housed within a sheared matrix.

The alignment of micas produces the strong mylonitic foliation, with lenses of

recrystallised quartz running sub-parallel to this.

2.5 Thin section work

The use of optical mineralogy aids interpretations into the nature of deformation

within the Paparoa Metamorphic Core Complex, through the study of microstructures.

Observations and interpretations into the ‘microtectonics’ of rocks at the thin section-

scale are made in reference to the work by Passchier and Trouw (1996).

Intracrystalline deformation is suggested by the presence of undulose extinction,

which develops in response to dislocations in the crystal lattice during deformation.

Deformation of feldspar twins may also occur during metamorphism, resulting in the

bending and/or fracturing of twin planes.

Thin sections were cut from hand specimens RS-35 to RS-40 and PCC06-01, with

sections being cut perpendicular to foliation (if present). Estimated percentages of

mineralogy are listed in Table 2.1, with observations from each sample being

discussed below.

66

RS-35 RS-36 RS-37

Chlorite: 30% Quartz: 40% Quartz: 30%

Quartz: 25% Chlorite: 10% Chlorite: 30%

Alk. Feldspar: 15% Biotite: 20% Alk. Feldspar: 25%

Plag. Feldspar:15% Alk. Feldspar: 10% Plag. Feldspar:10%

Biotite: <5% Plag. Feldspar:10% Muscovite: <5%

Zircon: <5% Muscovite: <5% Biotite: <1%

Muscovite: <5% Zircon: <5% Magnetite: <1%

Magnetite: <1% Magnetite: <1% Zircon: <1%

RS-38 RS-39 RS-40

Quartz: 25% Quartz: 30% Quartz: 40%

Chlorite: 20% Alk. Feldspar: 25% Alk. Feldspar: 20%

Alk. Feldspar: 25% Plag. Feldspar:20% Plag. Feldspar:15%

Plag. Feldspar:10% Biotite: 10% Chlorite: 10%

Muscovite: 10% Muscovite: 10% Muscovite: 10%

Biotite: <5% Chlorite: <5% Biotite: <5%

Magnetite: <5% Magnetite: <1% Zircon: <1%

Zircon: <1% Zircon: <1%

PCC06-01

Quartz: 30%

Biotite: 30%

Plag. Feldspar:15%

Alk. Feldspar: 10%

Muscovite: 10%

Zircon: <5%

Magnetite: <1%

Table 2.1: Percentage estimates of thin sections for hand specimens RS-35 to RS-40 from the Lower Buller Gorge area and hand specimen PCC06-01 from the coastal section north of Fox River Mouth.

67

Sample RS-35 (Figure 2.8a and 2.8b):

Chlorite comprises a large percentage of the total minerals present, resulting from

the alteration of biotite. Chlorite grains are subhedral in shape.

Quartz minerals have experienced dynamic recrystallisation, and display both

undulose extinction and fluid inclusion trails. Quartz grains are anhedral in shape.

Feldspar minerals present include alkali- and plagioclase feldspar, with both albite

and pericline twins observed. Some feldspar grains exhibit undulose extinction and

fluid inclusion trails, however, no deformation of twinning is observed. Feldspar

grains are subhedral to anhedral in shape.

Minor amounts of biotite and muscovite are present, which are both anhedral in

shape. A small percentage of magnetite and zircon grains are present.

This specimen is largely undeformed at the thin section scale, with an overall seriate-

interlobate grain distribution pattern (I in the deformation classification scheme).

Sample RS-36 (Figures 2.8c and 2.8d):

Quartz grains are equigranular to anhedral in shape, with undulose extinction and

fluid inclusion trails present. Obvious dynamic recrystallisation has occurred.

Chlorite grains are acicular in shape, and have developed in response to the

alteration of biotite. Acicular grains of biotite are reddy-brown in appearance, and are

aligned in an obvious foliation.

Feldspars present include alkali- and plagioclase feldspar, which are anhedral in

shape. Feldspar minerals are very fine grained in response to extensive dynamic

recrystallisation. Anhedral muscovite grains are accompanied by a small number of

zircon and magnetite minerals.

68

Figure 2.8a: Thin section image of RS-35 at 2.5x mag. Scale bar: 1cm. CPL. Note the limited deformation (dynamic recrystallisation of quartz grains).

Figure 2.8b: Thin section image of RS-35 at 2.5x mag. Scale bar: 1cm. CPL. Note the limited deformation (dynamic recrystallisation of quartz grains).

69

Figure 2.8c: Thin section image of RS-36 at 2.5x mag. Scale bar: 1cm. CPL. Note the dynamic recrystallisation of quartz and feldspar and the beginnings of foliation development

Figure 2.8d: Thin section image of RS-36 at 10x mag. Scale bar: 1cm. CPL.

70

This specimen appears to have experienced a small degree of deformation, resulting

in the inequigranular, fine grained appearance of the sample, and the beginnings of

foliation development (II in the deformation classification scheme).

Sample RS-37 (Figures 2.8e and 2.8f):

Quartz grains are anhedral in shape, with undulose extinction, fluid inclusion trails,

and evidence of dynamic recrystallisation.

Alkali- and plagioclase feldspar minerals show both undulose extinction and fluid

inclusion trails, with a subhedral grain shape. The larger relic grains show evidence of

recrystallisation, and overprinting and/or replacement by rods of muscovite in a

myrmekitic fashion. Albite and pericline twinning present, with no obvious

deformation of twins.

Chlorite grains are subhedral to anhedral in shape, with undulose extinction. Mica

grains show subhedral to anhedral patterns, with the biotite having a reddy-brown

appearance. Minor amounts of magnetite and zircon are present.

This specimen is seriate-interlobate in nature, and is considered to be relatively

undeformed (I in the deformation classification scheme).

Sample RS-38 (Figures 2.8g and 2.8h):

Quartz grains show evidence for dynamic recrystallisation, with undulose extinction,

fluid inclusion trails and quartz ribbons present. Grains are anhedral in shape.

Chlorite and biotite minerals make up the foliation, and are closely associated due to

the process of alteration. Grain shapes range from subhedral to acicular in appearance.

Muscovite grains are acicular in shape, aligned parallel to the foliation.

71

Figure 2.8e: Thin section image of RS-37 at 2.5x mag. Scale bar: 1cm. CPL. Note dynamic recrystallisation of quartz, and overprinting and/or replacement by rods of muscovite in a myrmekitic fashion.

Figure 2.8f: Thin section image of RS-37 at 2.5x mag. Scale bar: 1cm. CPL. Note dynamic recrystallisation of quartz and development of quartz ribbons.

72

Figure 2.8g: Thin section image of RS-38 at 2.5x mag. Scale bar: 1cm. CPL. Note the distinct foliation, dynamic recrystallisation of quartz and feldspar grains and the development of quartz ribbons.

Figure 2.8h: Thin section image of RS-38 at 10x mag. Scale bar: 1cm. CPL.

73

Feldspar minerals are very fine in grain size, due to the influence of dynamic

recrystallisation. No twinning is observed due to the fine grain size.

Small percentages of magnetite and zircon grains are present.

This specimen shows a substantial amount of recrystallisation, with the development

of an obvious foliation in an inequigranular fashion (II in the deformation

classification scheme).

Sample RS-39 (Figures 2.8i and 2.8j):

Quartz minerals are observed to consist of fine grained areas in which dynamic

recrystallisation has occurred, and in the larger, less deformed quartz ribbons. Grains

are largely anhedral in shape, with undulose extinction and fluid inclusion trails

present.

Feldspar minerals include both alkali- and plagioclase feldspar, which are subhedral

to anhedral in shape. The feldspars also show fine grained areas associated with

dynamic recrystallisation, and larger relic grains with less extensive deformation.

Plagioclase minerals display myrmekitic textures by the inclusion of thin rods of

quartz within the relic grains. No deformation of twins observed.

Biotite grains are subhedral to anhedral in shape, and have a reddy-brown

appearance. Muscovite grains have subhedral to acicular patterns, with the beginnings

of mica fish development.

Chlorite is relatively sparse in this sample, as little observable alteration has taken

place. Grains are subhedral to acicular in shape. Minor amounts of magnetite and

zircon are present.

This specimen shows some degree of deformation, with dynamic recrystallisation

and undulose extinction observed in most minerals present. The overall pattern of

74

Figure 2.8i: Thin section image of RS-39 at 2.5x mag. Scale bar: 1cm. CPL. Note the dynamic recrystallisation of quartz and feldspar, development of quartz ribbons, and the myrmekitic texture of the feldspar due to the inclusion of quartz rods.

Figure 2.8j: Thin section image of RS-39 at 2.5x mag. Scale bar: 1cm. CPL.

75

grain shapes is seriate-interlobate in nature (II in the deformation classification

scheme).

Sample RS-40 (Figures 2.8k and 2.8l):

Quartz grains are anhedral in shape, with elongate subgrains and fluid inclusion

trails present. Dynamic recrystallisation and undulose extinction present.

Feldspar minerals show undulose extinction, dynamic recrystallisation and fluid

inclusion trails. Both alkali- and plagioclase feldspar minerals are present, with

anhedral grain shapes. Deformed albite and pericline twins are observed, with flame

shaped albite lamellae common.

Chlorite grains are subhedral to anhedral in shape. Large muscovite grains are

acicular in shape, with the beginnings of mica fish. Biotite grains present are anhedral

in shape; however, the majority of grains have been altered to chlorite. Minor

amounts of zircon, with no opaques observed.

This specimen is seriate-interlobate in nature, and is considered to have experienced

some degree of deformation. Dynamic recrystallisation and undulose extinction is

observed in most minerals, with flame shaped albite lamellae and the initiation of

mica fish growth also illustrating the degree of deformation (II in the deformation


Sample PCC06-01 (Figures 2.8m and 2.8n):

Quartz minerals display undulose extinction, fluid inclusion trails and evidence for

dynamic recrystallisation. Grains are anhedral in shape, with quartz also existing in

ribbons.

Reddy-brown biotite grains are subhedral to acicular in shape, aligned in a foliation.

76

Figure 2.8k: Thin section image of RS-40 at 2.5x mag. Scale bar: 1cm. CPL. Note the dynamic recrystallisation of quartz and feldspar, the deformation of both albite and pericline twins and the beginnings of mica fish.

Figure 2.8l: Thin section image of RS-40 at 2.5x mag. Scale bar: 1cm. CPL. Note the presence of flamed shaped lamellae in the feldspar grains and the extensive dynamic recrystallisation of quartz and feldspar.

77

Figure 2.8m: Thin section image of PCC06-01 at 2.5x mag. Scale bar: 1cm. CPL. Note the extensive dynamic recrystallisation of quartz and feldspar, the growth of muscovite ribbons and mica fish.

Figure 2.8n: Thin section image of PCC06-01 at 2.5x mag. Scale bar: 1cm. CPL.

78

Feldspars include both alkali- and plagioclase feldspar, which are anhedral in shape.

Both albite and microcline twins present, with evidence of deformation by the

bending of twins. Muscovite grains are acicular in shape, and are present in ribbons

parallel to the foliation and as mica fish. Minor amounts of zircon and magnetite

present, with no observable chlorite due to a lack of alteration.

Quartz grains commonly deform plastically in mylonitic rocks, with recrystallised

quartz squeezing between the more robust feldspar grains. Feldspars often deform in a

brittle fashion in mylonitic rocks, with the disintegration of sharp grain boundaries

and fracturing of grains being common (Tulloch and Kimbrough 1989). This

behaviour is observed both in previous studies on the Paparoa Metamorphic Core

Complex, and in this sample (PCC06-01).

This specimen is seriate-interlobate in nature, and is considered to have experienced

a high degree of deformation associated with mylonitisation (IV in the deformation


Lower Plate rocks adjacent to the Ohika Detachment Fault (samples RS-35 to RS-

40) experienced limited recrystallisation of quartz, with deformation predominantly

cataclastic in nature.

The Lower Plate rock adjacent to the Pike Detachment Fault (sample PCC06-01),

however, shows greater levels of ductile deformation, with the recrystallisation of

quartz and development of mica fish.

79

CHAPTER 3: ZIRCONOLOGY

3.1. Use of zircon in geochronology

Petrogenetic studies have recently shifted from the whole rock approach, in favour of

analysing individual mineral groups. The advantages and disadvantages for each

approach have been summarised by Amelin and co workers (2000), as shown in Table

3.1.

Zircon minerals are a valuable tool in petrogenetic investigations, through textures

recorded within the minerals, and the overall morphology of the grains within samples.

Zircon (ZrSiO4) consists of various elements which are of interest in geochemical studies,

including U, Pb, Th, Hf, P, Y, and the Rare Earth Elements (REE’s). A solid solution

series exists between ZrSiO4 and HfSiO4, with (Hf, Zr)SiO4 with more than 90 mol.-%

hafnium component being referred to as hafnon (Neves et al. 1974). The chemical

structure of zircon is discussed in detail by Finch and Hanchar (2003) and references

therein.

The development of crustal material from the mantle may be investigated through the

interpretation of Hf isotopes, while oxygen isotopes highlight the presence or absence of

a sedimentary source (Hawkesworth and Kemp 2006, Kemp et al. 2006).

Titanium has recently become an important tool in the calculation of zircon

crystallisation temperatures during magmatic and/or metamorphic events (Valley 2003,

Watson and Harrison 2005), with temperature and melt composition influencing the

solubility of titanium (Hayden and Watson 2007). By using titanium content as a proxy

80

Complexity of geochemical systems

Variable. Small size allows better chances of finding simple systems

Very complex. Assembly of multiple phases with various ages and evolution paths

Diagnostics of components with various ages and origins

U-Pb dating using microbeam techniques or crystal fragments; Imaging

Indirect evidence from geological relationships

Closed versus open systems: Geochemical check

Monitoring closed system behaviour of the U-Pb. Extrapolation to other systems assumes slow diffusion of elements

Closed system assumption is as good as the isochron fit (if the other conditions of isochron model are satisfied). Comparison between different isotopic systems is possible but may not be straightforward

Prospect for the data improvement with the advancement of analytical techniques

Increasing sensitivity of isotopic analysis and better methods for cutting mineral grains can greatly enhance the data quality

Improving analytical precision would be useful. Sensitivity is relatively less important for most rocks

Mineral grains and grain fragments

Whole rocks

Closed verses open systems: Additional ways of checking

Imaging: CL, BSE, optical microscopy of etched surface, etc.

Several lines of indirect evidence: deformation textures, presence of secondary minerals, changes in rock chemistry

Dating options Direct precise dating with U-Pb Isochron dating – relatively imprecise and involves assumptions difficult to verify

Sample preparation Complex, time-consuming, and requires substantial experience

Relatively simple and straightforward

Difficulties Mainly analytical (handing and precise analysis of very small mineral grains and fragments)

Mainly geochemical (related to the complexity of the system)

Table 3.1: Comparisons between the use of individual minerals and whole rocks in geochemical investigations (Amelin et al. 2000).

81

for temperature, links between metamorphic facies, geochemistry and geochronology

may be made.

The resilience of zircon is reflected in its ability to endure magmatic and metamorphic

events, and withstand the processes of erosion and chemical alteration. It is due to this

resilience that older xenocrystic cores may be inherited into younger rock units, thus

providing clues to the provenance of the rock unit (Black et al. 1986, Compston and

Pidgeon et al. 1986, Corfu et al. 2003).

3.2. Zircon morphology

Zircons can be classified on the basis of observations relating to their crystal shape,

inherited make-up and origin (e.g. metamorphic, igneous, hydrothermal or inherited).

Observations of variations (or absences) in crystal shape, growth zoning, alteration and

metamorphism may be made through using cathodoluminescence (CL) or back-scattered-

electron (BSE) imaging, inferred through U-Pb geochronology.

3.2.1. Crystal shape

Simple observations of zircon crystals using CL or BSE images provide clues to the

evolution of individual grains within rock units, and therefore an appreciation of its

geological history and provenance.

The external nature of zircons often reflects the conditions in which it has existed.

Rounded grains may be indicative of shearing associated with metamorphism, erosion or

resorption, while more angular grains often reflect primary crystal growth which has not

experienced much disturbance (Hoskin and Schaltegger 2003). An overall decrease in

82

grain size is commonly observed in zircons that have experienced high levels of strain,

such as those contained in mylonitic shear zones (Wayne and Sinha 1988).

3.2.2. Primary oscillatory growth zoning

Internal features, especially the nature of their contacts, also aid investigations into

geochronology. Growth zones are made visible through CL and BSE imaging, with band

thicknesses (measured in μm), inherited xenocrystic cores and younger overgrowth rims

being highlighted by such imaging techniques. Zoning patterns develop in response to

variations in the concentrations of Zr, Hf, P, Y, REE’s, U and Th, due to the low

diffusion rates and relative immobility of such elements (Belousova et al. 2006).

Mattinson and co workers (1996) suggest growth zones develop through the interaction

of crystal growth, diffusion rate, the relationship between the solid-liquid interface, and

the extent of supersaturation of the melt. Primary oscillatory growth zoning can therefore

be used as a proxy for intracrystalline diffusion, and may also record the relative degree

of zircon-saturation of a melt. Zircon grains with widely spaced zoning patterns suffering

from dissolution commonly characterize melts undersaturated in zircon, while the more

narrowly spaced zoning patterns with little to no dissolution commonly develop in

zircon-saturated melts (Vavra 1990).

Different periods of zircon growth often vary chemically, and therefore emit different

colours in both CL and BSE imaging techniques. In general, dark zones in CL (light in

BSE) represent U- and Th-rich sections; whereas light zones in CL (dark in BSE)

generally represent U-poor and REE-rich sections (Corfu et al. 2003).

83

Contacts between inherited xenocrystic cores and overgrowth rims may be truncated in

various fashions, and ‘ghost’ textures may develop through the preservation of primary

oscillatory growth zoning during later metamorphism. The presence of ‘ghost’ growth

zones in zircon illustrates the minerals ability to retain chemical histories, despite

suffering from partial recrystallisation (Hoskin and Black 2000).

3.2.3. Alteration and metamorphism

The destruction of primary oscillatory zoning patterns in grains is thought to take place

during recrystallisation of zircon under solid state conditions, through increased strain

within the mineral lattices at low temperatures (Köppel and Sommerauer 1974, Hoskin

and Black 2000). Brittle deformation of individual grains, and subsequent alteration

and/or Pb-loss, may take place at temperatures beneath the 600-650°C annealing

temperature of zircon (Mezger and Krogstad 1997).

Solid-state recrystallisation may result in a number of deformation textures, including

blurred primary zoning, convoluted zoning, and ‘ghost’ textures through transgressive

recrystallisation (Hoskin and Schaltegger 2003 and references therein).

Zircon minerals may also become altered through the resorption of original growth

zones, which may represent phases of zircon undersaturation in the melt. Once resorbed,

zircon may recrystallise, often resulting in the truncation and overlapping of any

remaining primary textures. Multiple discontinuities may develop in response to a

number of processes, such as the incorporation of several xenocrystic cores, resorption of

both primary and secondary structures, or several episodes of resorption during a single

magmatic event (Corfu et al. 2003).

84

3.2.4. Pb-loss

The loss of Pb in zircons is thought to develop through the mobilisation of Pb during

metamictization (Mezger and Krogstad 1997, Hoskin and Black 2000). Metamictization

involves the complete destruction of mineral lattices as a result of the radioactive decay

of U and Th, to produce metamict material (Woodhead et al. 1991). The external

morphology of such material remains undamaged, while the internal lattice commonly

experiences volume expansion and associated fracturing. Fracturing allows fluid to move

through the metamict material, thus promoting further damage to the mineral lattice

(Hoskin and Schaltegger 2003).

Volume expansion of xenocrystic cores occurs through the radioactive decay of U,

resulting in radial cracking and the fracturing of brittle low-U rims. Conversely, zircons

with low-U xenocrystic cores are unlikely to experience expansion-related fracturing,

leaving high-U rims to darken (in CL) relative to the xenocrystic cores. This method of

alteration results in the hydration of zircon, with a general increase in Fe and Ca and

reduction in Pb (Wayne and Sinha 1988).

Processes involved in the loss of Pb from zircons have been discussed by Mezger and

Krogstad (1997), who propose four main methods:

1. recrystallisation of metamict zircon, resulting in the mobilisation of Pb,

2. Pb diffusion in metamict zircon without the process of recrystallisation,

3. leaching of Pb from metamict zircon via fluids, and

4. diffusion of Pb in pristine zircon.

Diffusion of pristine zircon, however, does not develop under most crustal conditions,

as it does not involve the process of metamictization. Diffusion in metamict zircon is

85

considered the most likely method of Pb-loss, as mineral lattices with high levels of

radiation-related damage encourage high rates of diffusion (Cherniak et al. 1991). Pb-loss

associated with the crystallisation of new zircon, or recrystallisation of existing grains,

generally takes place under amphibolite- to granulite-facies metamorphic conditions

(Hoskin and Black 2000). Non-metamict zircon may potentially experience Pb-loss by

volume diffusion via fractures within the mineral lattice, under similar metamorphic

conditions (Ashwal et al. 1999). Significant levels of Pb-loss by volume diffusion,

however, are considered to be highly unlikely due to the extremely slow rate of diffusion

in non-metamict zircons shown in laboratory simulations (Cherniak and Watson 2003).

Metamict zircons are more likely to suffer from Pb-loss through diffusion, due to

increased diffusion rates through radioactively-damaged mineral lattices (Cherniak et al.

1991).

Any evidence of alteration obtained from CL imaging is clearly of great importance, as

geochemical analyses from such grains would be considered unreliable for interpretation

(Corfu et al. 2003).

3.3. Morphology of zircons from the PMCC

The morphology of zircons from samples RNZ119, UC08368, UC05592 and PCC06-01

are shown in Figures 3.1a, 3.1b, 3.1c and 3.1d, and discussed below. Uranium content has

been used to link the geochemistry of zircons with CL imaging, and is described as either

‘high’ or ‘low’ in content. For the purpose of this thesis, high-uranium content includes

all concentrations greater than 1000ppm, while low-uranium content refers to values

lower than 100ppm (Tables 3.2a, 3.2b, 3.2c and 3.2d; Mezger and Krogstad 1997).

86

Figure 3.1a: Cathodoluminescence images of individual zircon grains from sample RNZ119. Grains from left to right: B2-1, B2-2, B2-3, B2-4, B2-5 and B2-6. Scale bars: 50 µm in length. Geochemical sampling locations shown as circles for oxygen (solid lines), hafnium (short dashes) and U-Pb + trace elements (long dashes).

Figure 3.1b: Cathodoluminescence images of individual zircon grains from sample UC08368. Grains from left to right: 68-4, 68-5, 68-1, 68-3 and 68-2. Scale bars: 50 µm in length. Geochemical sampling locations shown as circles for oxygen (solid lines), hafnium (short dashes) and U-Pb + trace elements (long dashes).

87

Figure 3.1c: Cathodoluminescence images of individual zircon grains from sample PCC06-01. Grains from left to right: S-1, S-2, S-3, S-4, S-8, S-7, S-6, S-5, S-9, S-10 and S-11. Scale bars: 50 µm in length. Geochemical sampling locations shown as circles for oxygen (solid lines), hafnium (short dashes) and U-Pb + trace elements (long dashes).

Figure 3.1d: Cathodoluminescence images of individual zircon grains from sample UC05592. Grains from left to right: 92-1, 92-3, 92-2 and 92-4. Scale bars: 50 µm in length. Geochemical sampling locations shown as circles for oxygen (solid lines), hafnium (short dashes) and U-Pb + trace elements (long dashes).

88

Analysis C vs. R

208Pb corr'd 206Pb/238U age (Ma)

observed ±1s.e. U (ppm) ± Th (ppm) ± Th/U

BUCK_01 C 373.71906 1.88504915 775 44.6 68.1 4 0.09BUCK_02 R 111.1498235 0.451633494 542.3 2.9 72.4 0.5 0.13BUCK_03 C 112.5138063 0.718688839 314.1 4.7 118.7 2.3 0.38BUCK_04 R 135.4818137 1.05858967 158.3 1.4 72.5 0.7 0.46BUCK_05 C 110.4814407 0.901695887 248.7 4 267.2 5.8 1.07BUCK_06 R 103.4311859 0.637038014 1486.1 41.7 223.5 9 0.15BUCK_07 R 562.5694685 14.85492491 939.9 46.4 28.8 1.6 0.03BUCK_08 C 1604.237201 29.27925056 330.6 13.1 116.7 7.1 0.35BUCK_09 C 636.0121967 5.858756696 252.4 7.6 46.3 1.7 0.18BUCK_10 R 101.3314408 0.479427994 3492.2 25.3 87.1 1 0.02BUCK_11 C 314.6349783 17.57113524 318.3 5.6 71.5 1.6 0.22BUCK_12 C 116.3241816 1.712684739 93.3 5.2 28.2 1.7 0.3BUCK_13 R 103.2511763 0.519606292 2117.3 79.7 154.5 14.7 0.07BUCK_14 C 102.7025288 0.415364854 976.9 7.8 223.9 2.8 0.23BUCK_15 R 109.3607797 0.63347052 696.4 31.4 94.7 5.6 0.14BUCK_16 C 109.4416091 0.457400445 439.1 4.3 137.9 1.4 0.31BUCK_17 R 110.6759035 0.678463122 457.5 8.5 109.1 3.7 0.24BUCK_18 C 111.7369862 0.735415838 145.5 1.1 94.7 0.8 0.65BUCK_19 R 108.060718 0.958843766 188.3 5.3 135.6 4.3 0.72BUCK_20 R 122.695243 1.321720263 1229.9 24.5 109 2.7 0.09BUCK_21 R 118.7945414 1.280689392 144 7.3 98.8 5.1 0.69BUCK_22 C 550.1788569 7.688484969 202.6 1.9 84.8 2 0.42BUCK_23 C 470.4739599 3.90259659 1164.2 34.6 613.9 19.2 0.53BUCK_24 R 108.9868464 1.032028442 1820.6 41.8 116.6 3.2 0.06BUCK_25 C 1071.627117 4.752595087 94.6 0.9 54 0.7 0.57BUCK_26 R 1083.140186 4.679171728 154.9 1.7 57.6 0.8 0.37BUCK_27 R 101.7708905 0.493086322 824.8 7.2 122.8 1.3 0.15BUCK_28 C 276.8368327 5.100204643 188.2 4.6 42.6 1.2 0.23BUCK_29 C 228.5945607 1.848285917 391.7 9.7 325.8 9.7 0.83BUCK_30 R 77.27770481 0.770636644 5484.4 89.8 1567.4 29.4 0.29BUCK_31 C 239.1498642 2.945583965 2872 80.8 496.2 21.9 0.17BUCK_32 R 301.3601198 2.972386859 2942 75 153.2 7.2 0.05BUCK_33 C 520.7914135 18.42075521 1121.8 29.6 74.7 4.7 0.07BUCK_34 C 103.3208177 1.51073136 3219.4 37.6 1102.5 14.6 0.34BUCK_35 R 101.74642 0.747147447 1339 32.6 296.1 7.6 0.22

Table 3.2a: U, Th and Th/U ratios of zircon grains with 208Pb corrected 206Pb/238U ages from sample RNZ119. Alternating font style (bold vs. regular) show analyses common to individual zircon grains. Analyses in italics are sample spots which were not able to be distinguished as cores or rims.

89

AnalysisCores

vs. rims

208Pb corr'd 206Pb/238U age


68_01 C 0.07608141 0.0012919 506.4 31.6 94.3 7.3 0.1968_02 R 0.06297728 0.0006158 318.9 11.1 90.1 3.5 0.2868_03 C 0.17666481 0.00173279 131.4 2.9 70.1 1.7 0.5368_04 R 0.16617933 0.00189755 220.1 2.9 136.4 3 0.6268_05 C 0.0798396 0.00089678 1141.4 19.1 377.9 12.8 0.3368_06 R 0.07119915 0.00076679 1376.1 30 341.4 12.3 0.2568_07 R 0.2062262 0.00348624 603.8 8.9 194.8 6.6 0.3268_08 R 0.0741271 0.0016922 1445.7 78.8 107.6 6.1 0.0768_09 C 0.12545889 0.00327354 326.3 10.7 61.7 3.4 0.1968_10 R 0.05230716 0.00064004 1239.2 51.2 44.1 1.9 0.0468_11 C 0.17352587 0.00284299 372.5 13.5 86 3.6 0.2368_12 R 0.15833916 0.00196004 832.3 25.8 146.8 5.5 0.1868_13 C 0.05740094 0.0002753 194.5 2.8 97 2.1 0.568_14 ? 0.06517195 0.00156919 2028.1 98.6 66.8 6.7 0.0368_15 ? 0.081862 0.00089336 1113.9 34 57.4 2.5 0.0568_16 ? 0.04756201 0.00047228 1699.7 12.7 26.2 0.3 0.0268_17 C 0.19215437 0.00105178 375 2.7 224.5 1.8 0.668_18 R 0.16091985 0.00299038 352.9 2.4 38.7 0.7 0.1168_19 C 0.21546371 0.00772619 287.6 7.3 111.2 3.9 0.3968_20 R 0.05335202 0.00033827 1398.1 35.1 100.9 2.8 0.0768_21 C 0.10709415 0.00364908 612.9 32.9 128.4 11.5 0.2168_22 R 0.15350531 0.00149673 180 6.2 89.2 4.2 0.568_23 ? 0.16727636 0.00120705 336.8 7.3 154.5 4.4 0.4668_24 ? 0.08179109 0.00090845 1238.1 21.1 36 1.3 0.0368_25 ? 0.05836476 0.00046647 314.2 16.2 109.7 6.9 0.3568_26 ? 0.05435531 0.00025136 555.9 32.8 113.3 8.2 0.268_27 ? 0.06342026 0.00041078 402.1 19.3 121.8 5.9 0.368_28 ? 0.0508367 0.00019292 2225.1 24.6 99.2 4.5 0.0468_29 ? 0.08962102 0.00266691 981 47.2 177.1 13.3 0.1868_30 ? 0.05067122 0.00039926 2031.5 28.8 59.5 1.8 0.03

Table 3.2b: U, Th and Th/U ratios of zircon grains with 208Pb corrected 206Pb/238U ages from sample UC08368. Alternating font style (bold vs. regular) show analyses common to individual zircon grains. Analyses in italics (“?”) are sample spots which were not able to be distinguished as cores or rims.

90

AnalysisCores

vs. rims



S_01 C 1473.462613 6.627419359 238 3 106.2 1.8 0.45S_02 R 1413.899969 6.78040733 1120.6 23.5 285.7 7.1 0.25S_03 C 1378.486856 9.219342881 544.3 11 93.3 3.9 0.17S_04 R 1203.748023 15.5701138 825.8 9.6 62.9 4.5 0.08S_05 C 596.4776804 3.502823746 834.3 14.4 334.6 6.9 0.4S_06 R 592.8904502 1.983523096 528.4 6.8 154.6 2 0.29S_07 R 352.904914 1.687624792 658.3 11.3 8.9 0.2 0.01S_08 R 610.3007943 13.47716906 384.9 9.5 59.6 3.7 0.15S_09 R 338.6017125 1.404528002 636.5 2.8 5.5 0.2 0.01S_10 C 1000.848893 3.72274997 328.4 3.3 225.8 2.5 0.69S_11 C 1170.802297 33.13401823 96.5 4.3 52.5 3.3 0.54S_12 C 995.0984611 7.524167576 1837.7 26.5 116.4 5.9 0.06S_13 R 659.88117 4.272457158 1085.5 18 93.5 2.4 0.09S_15 R 762.1101741 10.96420316 584.9 9.6 223.1 8.4 0.38S_16 C 905.0466005 5.031039359 640.2 3.9 278.5 2.8 0.44S_17 R 529.705269 4.204342851 575.1 9.1 43.2 1 0.08S_18 R 782.9927549 30.01356276 2936.5 117.9 367.3 40.4 0.13S_19 C 697.1386036 9.95563885 1190.1 57.4 148 15.8 0.12S_20 R 462.0378448 3.231204731 731.1 16.4 29.6 0.8 0.04S_21 C 529.9996739 2.838762267 2033.4 31.9 142 2.7 0.07S_22 C 865.339485 14.69212228 333.2 22 56.4 4.7 0.17S_23 R 562.758152 3.194642802 1208.7 8.5 32.8 0.4 0.03S_24 C 595.0140087 4.861365097 180.2 2.6 45 2.7 0.25S_25 R 381.0645092 3.749285606 409.2 14.4 17.2 1 0.04S_26 C 919.9552513 3.791293593 551.9 6.8 362.3 4.8 0.66S_27 R 442.6680036 2.40193608 507.9 8.3 33 1.5 0.06S_28 C 621.3486368 3.077654289 1894.2 31.4 109.8 3.2 0.06S_29 R 486.1675665 5.777617644 364.1 11.3 158.8 9.2 0.44S_30 C 1954.576378 8.222018884 359.8 5.8 315.7 6.6 0.88S_31 R 674.4936096 13.86225464 513.2 5.9 15.3 0.8 0.03S_32 C 550.1452145 3.289844363 130.1 1.3 53.7 0.6 0.41S_33 R 329.0483789 1.771434217 626.9 3.9 5.8 0.4 0.01S_34 C 924.6954943 6.204854501 120.4 2.9 30.4 0.8 0.25S_35 R 770.8304771 8.300412506 753.4 25.9 30.5 1.8 0.04

Table 3.2c: U, Th and Th/U ratios of zircon grains with 208Pb corrected 206Pb/238U ages from sample PCC06-01. Alternating font style (bold vs. regular) show analyses common to individual zircon grains. Analyses in italics (“?”) are sample spots which were not able to be distinguished as cores or rims.

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AnalysisCores

vs. rims



92_01 ? 297.4303486 3.991886304 860.6 7.8 217.1 3 0.2592_02 ? 239.5477162 3.886700438 1168 32.7 200.6 7.4 0.1792_03 C 336.0306656 2.080477828 629.7 15 281.9 13.6 0.4592_04 ? 761.1094961 9.247204587 3334.7 44.2 221.3 3.2 0.0692_05 C 1015.456051 4.461412297 3345.5 69.9 269.4 5.7 0.0892_06 R 160.9600057 6.355173783 2589.3 94.6 193.5 8.6 0.0792_07 R 249.3183688 13.86488785 1711.9 167.1 169.2 18.8 0.192_08 C 665.0654357 5.522177547 623.9 5.7 180.2 1.9 0.2992_09 R 781.3093172 6.247766846 3728.8 70 266.9 7.6 0.0792_10 R 293.3765261 1.784479247 595.6 10.5 529.2 10.8 0.8992_11 C 295.2686703 2.537155842 747.9 8.1 678.2 13.7 0.9192_12 R 329.0550824 5.364985881 914.6 11.7 245.9 8.8 0.2792_13 C 86.64086097 1.312660606 6814.6 152.8 201.3 14.6 0.03

Table 3.2d: U, Th and Th/U ratios of zircon grains with 208Pb corrected 206Pb/238U ages from sample UC05592. Alternating font style (bold vs. regular) show analyses common to individual zircon grains. Analyses in italics (“?”) are sample spots which were not able to be distinguished as cores or rims.

Aspect ratios of all zircon grains are listed in Table 3.3, with average aspect ratios shown

in Table 3.4.

3.3.1. RNZ119

Zircon grains from this sample are dominantly euhedral in shape, with aspect ratios

ranging from 1.6:1 to 3:1, with a mean of 2.5:1 (Table 3.4).

CL imaging shows a general trend of decreasing luminosity towards the crystal margins,

with ‘dark’ and ‘light’ growth zoning bands ranging between 5 to 10 µm in thickness.

92

93

Sample name Zircon grain Long/short axis (µm) Aspect ratio

PCC06-01 S_01 140/75 1.9:1S_02 175/150 1.2:1S_03 120/60 2:01S_04 75/60 1.25:1S_05 150/100 1.5:1S_06 150/125 1.2:1S_07 125/100 1.25:1S_08 125/75 1.7:1S_09 175/110 1.6:1S_10 110/75 1.5:1S_11 100/50 2:01

UC05592 92_01 150/100 1.5:192_02 110/75 1.5:192_03 150/75 2:0192_04 110/60 1.8:1

RNZ119 B_01 150/50 3:01B_02 150/50 3:01B_03 150/50 3:01B_04 300/125 2.4:1B_05 275/125 2.2:1B_06 200/125 1.6:1

UC08368 68_01 225/80 2.8:168_02 175/125 1.4:168_03 150/60 2.5:168_04 125/80 1.6:168_05 200/200 1:01

Table 3.3: Long/short axis and aspect ratios for zircons from the four samples from the Paparoa Metamorphic Core Complex.

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SampleZircon length range

(µm) Minimum aspect ratio Maximum aspect ratio Mean aspect ratio

RNZ119 150-300 1.6:1 3:01 2.5:1

UC08368 125-220 1:01 2.8:1 1.8:1

UC05592 100-150 1.5:1 2:01 1.7:1

PCC06-01 100-300 1.2:1 2:01 1.6:1

Table 3.4: Aspect ratios from samples RNZ119, UC08368, UC05592 and PCC06-01 from the Paparoa Metamorphic Core Complex. The range of zircon grain lengths (in µm), minimum, maximum and mean aspect ratios are shown.

The nature of zoning varies between grains, with angular terminations in B2-1, B2-2, B2-

5 and B2-6, and rounded in nature in B2-3 and B2-4.

The uranium content and luminosity of zircon grains in CL images follow the expected

pattern in most cases, with ‘light’ zones having low-uranium and ‘dark’ zones containing

high-uranium (Figures 3.1a and Table 3.2a). The exception within this sample is zircon

grain B2-6, which shows a ‘light’ xenocrystic core and ‘dark’ rim with higher uranium

within the central zone. The variation in uranium content between the xenocrystic core

and rim of this grain, however, is minimal.

Alteration of zircon grains within this sample includes cracking (B2-3 and B2-6) and

corrosion (B2-1, B2-2 and B2-4), as shown in Figure 3.2a.

3.3.2. UC08368

Zircon grains within this sample are euhedral and stubby in nature, with aspect ratios

ranging from 1:1 to 2.8:1, with a mean of 1.8:1 (Table 3.4).

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Figure 3.2a: Cathodoluminescence images of zircons from sample RNZ119 from the Paparoa Metamorphic Core Complex. Various morphologic features are shown, as follows: ‘C’ for corrosion; ‘CF’ for concentric fracturing and ‘SP’ for damage associated with sample processing.

The use of CL imaging highlights different zones of growth with average thicknesses of

5 to 10 µm under CL. Zircons from this sample show an overall reduction in luminosity

towards crystal edges.

Most zoning patterns present show truncations of central zones by subsequent growth

bands, as seen in grains 68-2 and 68-5. The preservation of oscillatory zoning patterns in

grains which have suffered solid-state recrystallisation produces ‘ghost textures’, as seen

in grain 68-5. Corrosion of grains may have taken place in grains 68-1 and 68-2 (Figure

3.2b).

96

Figure 3.2b: Cathodoluminescence images of zircons from sample UC08368 from the Paparoa Metamorphic Core Complex. Morphologic features are shown as follows: ‘C’ for corrosion and ‘GT’ for ‘ghost’ textures.

The uranium content and luminosity patterns in CL images correlate well in most grains

from this sample, with high-uranium content being associated with ‘dark’ zones and low-

uranium with ‘light’ zones (Figures 3.1b and Table 3.2b). Grains with high-uranium rims

and low-uranium xenocrystic cores include 68-2, 68-4 and 68-5, while 68-1 has low-

uranium rims (xenocrystic core not sampled) and 68-3 has low-uranium rims and a high-

uranium xenocrystic core.

3.3.3. PCC06-01

Zircon grains from this sample are subhedral and sub-rounded, in contrast to the more

elongate euhedral zircons from RNZ119, UC08368 and UC05592. Crystal edges are

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somewhat irregular, and stubby in nature, with aspect ratios ranging from 1.2:1 to 2:1,

and a mean of 1.6:1 (Table 3.4).

The relationship between uranium-content and degree of luminosity is consistent

throughout the sample, with high-uranium sections appearing ‘dark’, and low-uranium

sections ‘light’, in CL images. Grains with high-uranium xenocrystic cores and low-

uranium rims include S-1, S-10 and S-11, while the remainder show low-uranium

xenocrystic cores and high-uranium rims (Figure 3.1c).

Growth zones average between 5 to 10 µm in thickness, with bands seen in grains S-3,

S-4, S-7, S-8, S-10 and S-11. Angular terminations between banding and xenocrystic

core-rim divisions are common within this sample, which may be associated with

dissolution and/or corrosion. Dissolution textures are commonly observed in S-type

granitoid rocks, and develop through the shift from crystallisation in an initially zircon-

undersaturated melt to a zircon-saturated melt (Hoskin and Schaltegger 2003 and

references therein). Some grains show evidence of metamictization, by a ‘mottled’ or

‘crumbly’ appearance in CL images, as evident in the ‘dark’, uranium-rich xenocrystic

cores of S-1 and S-2 (Figure 3.2c).

3.3.4. UC05592

The crystal shape of zircons from this sample are euhedral and stubby with aspect ratios

ranging from 1.5:1 to 2:1, with a mean of 1.7:1 (Table 3.4).

An overall decrease in luminosity is observed from the centre to the edges of most

grains, with growth zones averaging between 5 to 10 µm in thickness. Growth zoning is

preserved in most grains, as seen in the xenocrystic core of 92-3, and the rims of 92-1 and

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Figure 3.2c: Cathodoluminescence images of zircons from sample PCC06-01 from the Paparoa Metamorphic Core Complex. Morphologic feature shown: ‘M’ for metamictization.

92-2. Overgrowth rims are clearly separate from the xenocrystic cores of grains in this

sample, by differences in luminosity in CL images and chemical compositions.

Uranium content and luminosity of grains follow the expected pattern, of ‘dark’ zones

consistent with high-uranium, and ‘light’ with low-uranium content. The majority of

grains are relatively uranium-rich in both the xenocrystic cores and rims, and show both

99

high-uranium rims (92-1) and high-uranium xenocrystic cores (92-2 and 92-3) (Figure

3.1d and Table 3.2d).

Three out of the four grains shown are fragmented and cracked, with at least one grain

(92-1) suffering from surficial damage associated with sample processing (Figure 3.2d).

3.4. Discussion of PMCC zircon morphology Interpretations of the morphology of zircon are based on information deduced from a

combination of CL images and chemical sampling.

Observations in zircon crystal shapes allow interpretations to be made about the overall

setting in which grains were produced. The growth of stubby and equant euhedral zircon

crystals is generally associated with slow-cooling of zircon-saturated intrusions at depth

(Corfu et al. 2003, Hoskin and Schaltegger 2003).

Zircon grains from the Buckland Granite (sample RNZ119) are dominantly euhedral in

shape, reflecting their slow crystallisation out of the melt during cooling of the pluton.

Zircon crystals from samples UC08368, UC05592 and PCC06-01 are dominantly stubby

in shape, which suggests the growth of such grains within deep seated plutonic rocks. The

rounding of zircon grains, as seen in sample PCC06-01, is thought to have resulted from

the sedimentary transport of crystals associated with the recycling of crustal material

since their initial growth in the parent rock(s). Metamorphic processes such as the

dissolution of growth terminations and shearing during active detachment faulting may

have also resulted in the rounding of grains.

100

Figure 3.2d: Cathodoluminescence images of zircons from sample UC05592 from the Paparoa Metamorphic Core Complex. Various morphologic features are shown, as follows: ‘C’ for corrosion; ‘CF’ for concentric fracturing; ‘GT’ for ‘ghost’ textures; ‘M’ for metamictization and ‘SP’ for damage associated with sample processing.

Aspect ratios involve the measurement of long and short axes of individual grains, and

allow quick comparisons of grain shapes to be made. The use of aspect ratios

complements observations in crystal shapes, to aid interpretations of crystallisation

conditions, as discussed above. Research into the response of zircons to increased strain

shows a systematic reduction in the grain size and aspect ratios of affected crystals (Sinha

and Glover 1978). Zircons from the Paparoa Metamorphic Core Complex display a

similar behaviour, with the smallest mean aspect ratio belonging to the sample PCC06-

01, collected from the Pike Detachment shear zone (Table 3.4).

Variations in the luminescence of zircon grains in CL images correspond with the

geochemical data, with ‘dark’ and ‘light’ zones commonly representing high- and low-

uranium contents, respectively. The distinction between xenocrystic cores and rims is

based on the morphology of individual grains, and interpretations of geochemical and

geochronological data.

101

The location of xenocrystic core-rim divisions is based upon variations in the

luminescence of grains in the CL images, which is clearly subject to interpretation.

‘Ghost textures’ in zircon grains develop through the preservation of growth zoning

during the process of resorption. Grain 68-5 of UC08368 is thought to display these

textures, and thus record the recrystallisation of zircon in the solid-state (Figure 3.2b).

The metamictization of zircon minerals involves the destruction of mineral lattices by

radioactive decay, resulting in a ‘mottled’ or ‘crumbly’ appearance in CL images. Zircon

grains S-1 and S-2 from sample PCC06-01 show evidence for metamictization, as shown

in Figure 3.2c. Fracturing of metamictized zircon may take place in response to

differences in the properties of uranium-rich and uranium-poor zones within individual

grains. Concentric fractures may develop along such boundaries, as seen in grain B2-6 of

sample RNZ119.

Volume expansion of uranium-rich zones in metamictized zircons may result in the

cracking of adjacent uranium-poor zones, allowing potential alteration within grains by

metamorphic fluids to take place. Grain B2-3 from the sample RNZ119 shows cracking

patterns, which could result from either volume expansion or damage associated with

sample processing. This grain has a high-uranium xenocrystic core and low-uranium

rims, however, which does not support the idea of volume expansion-related damage.

Fracturing of the grain B2-3 is therefore more likely to be due to damage associated with

sample processing.

Corrosion is evident in grains B2-1, B2-2 and B2-4 of sample RNZ119, and 68-1, 68-2

and 68-3 of sample UC08368, caused by the oxidisation of zircon. Pits seen on the

102

surface of grain 92-1 (of UC05592), and fragmentation of grains B2-5 (of RNZ119) and

92-1, 92-2 and 92-4 (of UC05592) is attributed to damage associated with the removal

and preparation of crystals for geochemical analysis.

The morphological features of zircon grains are complemented by both geochemical and

geochronological data, which allows greater interpretations into the petrogenesis of

igneous and metamorphic rocks to be made.

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CHAPTER 4: GEOCHEMISTRY AND GEOCHRONOLOGY

4.1. Geochemical investigations

The use of zircon in the interpretation of granitoid petrogenesis is extended through

the use of geochemistry. Initial geochemical sampling often involves whole rock

analysis, through X-ray fluorescence (XRF).

Oxygen isotopes and hafnium constrain the role of crustal contamination, allowing

interpretations into the petrogenesis of the Buckland Granite during its emplacement

into the Paparoa Metamorphic Core Complex. Caution is required when using

hafnium as an indicator of crustal contamination, as the crystallisation of zircons in a

melt may mix with inherited zircons from other sources, which separated from the

mantle at a different time in the evolution of the crust (Hawkesworth and Kemp

2006). A combined geochemical approach is clearly of great importance, with the use

of oxygen isotopes providing greater confidence in establishing the role of crustal

contamination in granitoid petrogenesis.

The concentration of titanium in zircon grains allows the calculation of

crystallisation temperatures, with the zircon saturation thermometry technique.

Rare Earth Elements (REE’s) help to further constrain the petrogenetic nature of rocks

from the Paparoa Metamorphic Core Complex.

4.1.1. Whole rock analysis (XRF)

Major and trace element concentrations of crushed hand specimens are measured

through the process of XRF. Data for major elements, trace elements or a combination

of both may be illustrated using bivariate diagrams. One of the most commonly used

types of bivariate diagrams today are Harker Diagrams, which usually have silica

plotted as the abscissa against major and trace elements. Patterns and trends in the

103

chemical makeup of individual samples are clearly shown in such diagrams, allowing

initial interpretations to be made.

Samples from the Paparoa Metamorphic Core Complex were analysed by XRF, for

both major and trace element concentrations. UC05592, PCC06-01, UC08368, and

RS-35 to RS-40 were crushed and analysed at the University of Canterbury in 2006

(Tables 4.1a and 4.1b). The XRF data for RNZ119 was obtained from earlier whole

rock analysis at the University of Canterbury for Muir and co workers (1997), with all

data listed in Tables 4.1c and 4.1d).

Bivariate diagrams for samples UC05592, PCC06-01, UC08368, RS-35 to RS-40

and RNZ119 are plotted with SiO2 vs. TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O,

K2O and P2O5 (Figure 4.1). These samples from the Paparoa Metamorphic Core

Complex show a relative decrease with increasing SiO2 for TiO2, MnO, Al2O3, MgO,

Fe2O3, CaO and P2O5, and a relative increase with increasing SiO2 for Na2O and K2O.

The use of whole rock analysis, however, is relatively limited in regards to

provenance studies as only the ‘end result’ of an evolved body of rock is accessible.

Clearly, the in situ sampling of individual minerals is the more favourable method for

obtaining information on the petrogenesis of a rock unit.

4.1.2. Oxygen isotopes

Oxygen isotopes are a useful geochemical tool for looking at potential crustal

contamination within magmas, and the influence of mantle-derived material in

magmatic systems. The sluggish diffusion rates of oxygen isotopes make them

especially useful in the interpretation of chemical changes within a body of rock over

time.

104

105

106

107

The oxygen system involves the three stable isotopes 16O, 17O and 18O, with the

18O/16O ratio measured with respect to SMOW (Standard Mean Ocean Water).

SMOW is the most commonly accepted international standard used in oxygen isotope

work, and was originally defined by Craig in 1961 using the National Bureau of

Standards sample NBS-1 by the following equation:

(18O/16O)SMOW = 1.008 (18O/16O) NBS-1

The current formula commonly in use for oxygen isotope work is as follows:

δ (18O/16O) = (18O/16O)sample – (18O/16O)SMOW

(18O/16O)SMOW

δ18O values calculated with this formula are expressed per mil (‰), with δ18O

values >6.5‰ representative of 18O-enriched material, and δ18O values <6.5‰ of

material depleted in 18O (Faure 1986). In general, mantle rocks have δ18O values of +5

to +6‰, while the continental crust lies between +10 to >+25‰. Igneous rocks

commonly lie between +5 to +15‰, with values >6.5‰ involving the incorporation

of supra-crustal, recycled components in the melt (Winter 2001, Valley 2003, Kemp

et al. 2006). The δ18O value of 5.3 ± 0.3‰ represents the peridotitic mantle, with

values greater than this having a distinct sedimentary signature (Eiler 2001, Valley

2003, Valley et al. 2005, Kemp et al. 2006). The nature of oxygen at the time of

zircon crystallisation is recorded in δ18O values, in grains that have not experienced

metamictization. δ18O values may be therefore linked to U-Pb ages, Hf and Ti, for

each individual rock sample (Valley 2003).

108

Oxygen isotope data were sampled in situ from specimens RNZ119 [n2056],

PCC06-01 [n2064], UC05592 [n2065] and UC08368 [n2068], using the SIMS

technique (Table 4.2). The standard δ 18O value used was 9.86‰, which lies within

error of the accepted SMOW value of 9.58‰ (Winter 2001). Average δ18O values for

each sample were calculated, +/- per mil to 2dp (Table 4.3). δ18O values yield good

positive averages for each sample, illustrating the relative enrichment of 18O to

SMOW.

RNZ119 [n2056] has the greatest margin of error for both xenocrystic cores and rims

(8.34 ± 0.36 and 8.47 ± 0.36, respectively); with PCC06-01 [n2064] having the

highest δ18O values for xenocrystic cores (9.66 ± 0.15), and UC08368 [n2068] for

rims (10.46 ± 0.16).

No outliers are considered to exist in the data, as all δ18O values lie between the +5

and +15‰ characteristic of most igneous rocks (Winter 2001), with error margins of

± 0.15 to 0.36 per mil. No anomalous features were observed in CL where sampling

took place, as any zircon grains containing visible inclusions or cracks were avoided

to reduce potential outliers in the data set (Figures 3.1a, 3.1b, 31c and 3.1d).

4.1.3. Titanium

Titanium is a major element in zircon which has recently become a useful

geochemical tool in the calculation of crystallisation temperatures of zircon-bearing

igneous rocks. Zircon saturation thermometry has developed in the last few decades,

with two main approaches.

109

Sample Analysis δ18O ± per mil Sample Analysis δ18O ± per mi

RNZ119 n2056_ox_1c 8.81 0.37 PCC06-01 n2064_ox_1c 8.69 0.16n2056_ox_1r 7.96 0.37 n2064_ox_1r 8.73 0.15n2056_ox_2c 8.78 0.36 n2064_ox_2c 7.55 0.17n2056_ox_2r 8.03 0.37 n2064_ox_2r 9.3 0.14n2056_ox_3c 8.49 0.36 n2064_ox_3r 8.83 0.16n2056_ox_3r 10.06 0.36 n2064_ox_3c 7.96 0.15n2056_ox_4c 10.26 0.36 n2064_ox_4c 11.21 0.15n2056_ox_4r 7.97 0.36 n2064_ox_4r 11.78 0.16n2056_ox_5c 7.5 0.36n2056_ox_5r 7.75 0.36 UC08368 n2068_ox_1c 7.21 0.19

n2056_ox_5rr 7.69 0.36 n2068_ox_1r 10.43 0.16n2056_ox_6c 8.68 0.36 n2068_ox_2c 7.05 0.14n2056_ox_6r 9.45 0.36 n2068_ox_2r 10.37 0.15n2056_ox_7c 5.85 0.36 n2068_ox_3c 8.84 0.14n2056_ox_7r 8.87 0.36 n2068_ox_3r 10.02 0.16

n2068_ox_4c 10.33 0.17UC05592 n2065_ox_1c 8.72 0.16 n2068_ox_4r 9.62 0.16

n2065_ox_2r 8.74 0.17 n2068_ox_5c 11.39 0.17n2065_ox_3c 8.51 0.16 n2068_ox_5r 11.87 0.18n2065_ox_3r 9.29 0.14n2065_ox_4c 9.05 0.16n2065_ox_4r 6.34 0.16

l

Table 4.2: Oxygen isotope data for samples RNZ119 (n2056), PCC06-01 (n2064), UC05592 (n2065) and UC08368 (n2068) from the Paparoa Metamorphic Core Complex.

Sample Average δ18O ± per mil Average δ18O ± per mil Average δ18O ± per mil[analysis number] (cores) (2dp) (rims) (2dp) (total) (2dp)

RNZ119 [n2056] 8.34 0.36 8.47 0.36 8.41 0.36

PCC06-01 8.85 0.16 9.66 0.15 9.26 0.16

UC05592 [n2065] 8.76 0.16 8.12 0.16 8.44 0.16

UC08368 [n2068] 8.96 0.16 10.46 0.16 9.71 0.16

Table 4.3: Average δ18O data with +/- per mil (2dp) for RNZ119 (n2056), PCC06-01 (n2064), UC05592 (n2065) and UC08368 (n2068).

110

The ‘old’ approach to zircon saturation thermometry involves the use of major

element concentrations by ‘whole rock’ analyses, based on hydrothermal experiments

between 750 and 1020˚C (Hanchar and Watson 2003 and references therein).

Compositional data of melts and their associated crystallised zircons are used in the

solubility model, which was introduced by Watson and Harrison in 1983 with the

following equation:

ln DZrzircon/melt = (-3.80 – [0.85 (M – 1)]) + 12900/T

Where DZrzircon/melt represents the ratio of zirconium in the solid to zirconium in the

melt; M represents the cation ratio: (Na + K + 2*Ca)/(Al*Si); and T is temperature

(Kelvin).

The ‘new’ approach to zircon saturation thermometry involves the in situ sampling

of titanium from individual zircon grains, in order to calculate crystallisation

temperatures (Watson et al. 2006). Titanium is a well constrained trace element in

zircon which is relatively unaffected by changes in pressure, and records temperature

changes over time by variations in its concentration under Ti-saturated conditions.

Crystallisation temperatures are calculated with the following formula, and usually

have error margins of less than ± 10˚C within the 400-1000˚C range (Watson et al.

2006):

T (°C)zircon = 5080 ± 30 – 273

(6.01 ± 0.03) – log (Ti)

111

This method of calculating temperatures is considerably more efficient than the

‘whole rock’ approach, and is becoming widely adopted in the scientific community.

Titanium content of zircon grains in samples RNZ119, UC08368, UC05592 and

PCC06-01 were analysed by LA-ICP-MS at the Australian National University

Research School of Earth Sciences in Canberra. Raw titanium data from LA-ICP-MS

are listed in Table 4.4.

Crystallisation temperatures have been calculated for the Buckland Granite

(RNZ119) with the ‘new’ zircon saturation thermometry approach, to estimate

temperature conditions during its emplacement into the Paparoa Metamorphic Core

Complex (Table 4.5a). The other three samples (UC08368, UC05592 and PCC06-01)

are not able to provide any information on crystallisation temperatures during the

Cretaceous, due to their emplacement pre-dating the onset of the regional-scale

extension of the continental crust.

The average crystallisation temperature of the Buckland Granite (RNZ119) was

calculated using all 35 analyses (including both xenocrystic cores and rims), at 697°C

(3sf), with a standard deviation of 54.6 (3sf).

Xenocrystic cores and rims of zircon grains were analysed separately for the

Buckland Granite sample (RNZ119), in attempt to determine differences in age and

temperature during crystal growth. δT (°C) was calculated by subtracting the

estimated temperatures of rims from the estimated temperatures of xenocrystic cores,

for each grain that was sampled (Table 4.5a). Positive δT values are indicative of

grains with older xenocrystic

112

Analysis C/R 49Ti (ppm) Analysis C/R 49Ti (ppm) Analysis C/R 49Ti (ppm) Analysis C/R 49Ti (ppm)BUCK_01 C 12.4505762 S_01 C 6.13279773 68_01 C 7.2751621 92_01 ? 5.19045061BUCK_02 R 5.46449378 S_02 R 3.58149235 68_02 R 6.9718059 92_02 ? 6.32271604BUCK_03 C 7.34289163 S_03 C 116.823362 68_03 C 15.4403668 92_03 C 10.1351718BUCK_04 R 4.91487438 S_04 R 4.447368 68_04 R 10.309374 92_04 ? 6.12474064BUCK_05 C 16.9924246 S_05 C 4.55021644 68_05 C 3.02027338 92_05 C 8.53398422BUCK_06 R 30.3653469 S_06 R 4.53062354 68_06 R 3.45456618 92_06 R 16.176449BUCK_07 R 3.37231522 S_07 R 2.96656859 68_07 R 4.51691887 92_07 R 5.03999167BUCK_08 C 7.58597952 S_08 R 10.8926193 68_08 R 3.35837087 92_08 C 4.7169596BUCK_09 C 12.4386343 S_09 R 2.40664106 68_09 C 5.7392717 92_09 R 9.27295862BUCK_10 R 4.19160108 S_10 C 10.6204462 68_10 R 3.64127513 92_10 R 27.6782022BUCK_11 C 6.63844782 S_11 C 11.0773172 68_11 C 6.44492435 92_11 C 6.84313387BUCK_12 C 5.76247482 S_12 C 8.45227391 68_12 R 19.3850417 92_12 R 5.40506217BUCK_13 R 2.46354315 S_13 R 5.12374529 68_13 C 13.7176361 92_13 C 34.3898434BUCK_14 C 8.82482737 S_15 R 4.30121848 68_14 ? 2.794775BUCK_15 R 6.81697016 S_16 C 5.25663973 68_15 ? 7.45304616BUCK_16 C 6.13070206 S_17 R 4.17381941 68_16 ? 4.16316984BUCK_17 R 15.0398032 S_18 R 17.1927495 68_17 C 9.37381595BUCK_18 C 8.20092837 S_19 C 338.482517 68_18 R 2.32423607BUCK_19 R 15.5382059 S_20 R 3.88436996 68_19 C 5.00670847BUCK_20 R 4.31481643 S_21 C 117.194074 68_20 R 5.3922307BUCK_21 R 8.08909817 S_22 C 3.62803598 68_21 C 3.68821368BUCK_22 C 4.13408424 S_23 R 4.27311604 68_22 R 17.5831998BUCK_23 C 4.49786203 S_24 C 13.1278789 68_23 ? 29.7943473BUCK_24 R 4.48319055 S_25 R 3.63314014 68_24 ? bdlBUCK_25 C 16.7101318 S_26 C 9.25974963 68_25 ? 6.24947728BUCK_26 R 12.2827945 S_27 R 3.23339699 68_26 ? 484.410963BUCK_27 R 5.09853367 S_28 C 7.94891407 68_27 ? 4.25543931BUCK_28 C 5.44973055 S_29 R 6.05693557 68_28 ? bdlBUCK_29 C 9.03373213 S_30 C 5.40372423 68_29 ? 20.7464822BUCK_30 R 33.3480631 S_31 R 3.86650967 68_30 ? 4.74840605BUCK_31 C 14.33371 S_32 C 21.7017824BUCK_32 R 14.6704514 S_33 R 57.4859607BUCK_33 C 3.07919183 S_34 C 8.73303043BUCK_34 C 6.23715787 S_35 R 4.61315236BUCK_35 R 3.94952321

Table 4.4: Titanium contents of RNZ119 (Buck), PCC06-01 (S), UC08368 (68) and UC05592 (92). Alternating font style (bold vs. regular) shows analyses common to individual zircon grains. Analyses in italics are samples spots which are not common to individual grains. ‘?’ = analyses which may not be distinguished between cores or rims; bdl = below detection level.

113

Ti-in-zircon δT (°C)T (°C) (C - R)

BUCK_01 C 373.43 1.9 12.450576 760.6105429 70.111612BUCK_02 R 110.98 0.5 5.4644938 690.4989309BUCK_03 C 112.43 0.7 7.3428916 714.5328101 32.3740501BUCK_04 R 135.25 1.1 4.9148744 682.15876BUCK_05 C 109.21 0.8 16.992425 789.8182829 -59.1835709BUCK_06 R 103.14 0.6 30.365347 849.0018538BUCK_07 R 537.44 14.2 3.3723152 653.6569557 63.5987075BUCK_08 C 1580.18 28.6 7.5859795 717.2556632BUCK_09 C 634.64 5.8 12.438634 760.5229054 90.6204834BUCK_10 R 101.45 0.5 4.1916011 669.902422BUCK_11 C 313.79 17.4 6.6384478 706.1952894BUCK_12 C 110.82 1.7 5.7624748 694.731312 63.565715BUCK_13 R 103.08 0.5 2.4635432 631.165597BUCK_14 C 102.36 0.4 8.8248274 730.101364 21.725977BUCK_15 R 108.83 0.6 6.8169702 708.375387BUCK_16 C 108.85 0.5 6.1307021 699.7161191 -78.4435213BUCK_17 R 110.2 0.7 15.039803 778.1596404BUCK_18 C 111.15 0.7 8.200928 723.8334875BUCK_19 R 107.22 0.9 15.53821 781.2483127BUCK_20 R 122.74 1.3 4.314816 672.1096543BUCK_21 R 118.46 1.2 8.0890982 722.6684709 -53.8150661BUCK_22 C 557.18 7.7 4.1340842 668.8534048BUCK_23 C 470.32 3.8 4.497862 675.2930395 0.2511122BUCK_24 R 108.78 1 4.4831906 675.0419273BUCK_25 C 1066.92 4.7 16.710132 788.2029755 28.8301231BUCK_26 R 1080.67 4.6 12.282795 759.3728524BUCK_27 R 101.55 0.5 5.0985337 685.0287766 5.2554973BUCK_28 C 275.69 5.1 5.4497306 690.2842739BUCK_29 C 227.41 1.7 9.033732 732.1180311BUCK_30 R 76.11 0.8 33.348063 859.1773898 -85.5405257BUCK_31 C 233.38 2.9 14.33371 773.6368641BUCK_32 R 300.95 3 14.670451 775.8160871 -128.7867827BUCK_33 C 510.91 18.2 3.0791918 647.0293044BUCK_34 C 103.09 1.5 6.2371579 701.1106577 35.7081605BUCK_35 R 101.26 0.7 3.9495232 665.4024972

49Ti (ppm)Analysis C vs. R


observed ±1s.e.

Table 4.5a: Zircon saturation thermometry of the Buckland Granite (RNZ119) using titanium content, based on the method outlined in Watson et al., 2006. Alternating font style (bold vs. regular) shows analyses common to individual zircon grains. Analyses in italics are samples spots which are not common to individual grains, and are not included in the δT (°C) calculations.

114

cores and younger rims, while negative δT values represent grains which have

‘younger’ xenocrystic cores with ‘older’ rims.

Both positive and negative δT values are found in the Buckland Granite sample

(RNZ119), with negative δT values resulting from either Pb-loss or the in situ

sampling technique. Pb-loss is considered to be unlikely, as no substantial evidence of

metamictization was observed in CL for RNZ119 zircons. The inversion of ages

within individual grains is suggested to have resulted from the presence of shallow

cores, which appeared to be large in the CL images. The in situ sampling technique

may have drilled through the shallow core material, into progressively younger zircon

growth rims. The averaging out of ‘core’ age data would lead to the core appearing

younger than the outer rim, and thus cause the inversion of ages. This is considered to

be the most likely suggestion of the two, and is proposed for the five zircon grains

with analysis numbers BUCK-05 and 06, BUCK-16 and 17, BUCK-21 and 22,

BUCK-30 and 31 and BUCK-32 and 33.

Crystallisation temperatures of both zircon cores and rims within the Buckland

Granite (RNZ119) are listed in Tables 4.5b and 4.5c, respectively. The average

crystallisation temperature of xenocrystic cores from sample RNZ119 is 723°C (3sf),

with a standard deviation of 40.9 (3sf). The average crystallisation temperature of

zircon rims from sample RNZ119 is 730°C (3sf), with a standard deviation of 67.8

(3sf). The difference in average temperatures between xenocrystic cores and rims of

sample RNZ119 is considered to be negligible.

Granitoids rich in inherited zircons are considered to be ‘cold temperature’, with

temperatures less than 800˚C. Inheritance-poor granitoids contain few to no zircons,

115

Ti-in-zircon T (°C)

BUCK_01 C 373.43 1.9 12.450576 760.6105429BUCK_03 C 112.43 0.7 7.3428916 714.5328101BUCK_05 C 109.21 0.8 16.992425 789.8182829BUCK_08 C 1580.18 28.6 7.5859795 717.2556632BUCK_09 C 634.64 5.8 12.438634 760.5229054BUCK_11 C 313.79 17.4 6.6384478 706.1952894BUCK_12 C 110.82 1.7 5.7624748 694.731312BUCK_14 C 102.36 0.4 8.8248274 730.101364BUCK_16 C 108.85 0.5 6.1307021 699.7161191BUCK_18 C 111.15 0.7 8.2009284 723.8334875BUCK_22 C 557.18 7.7 4.1340842 668.8534048BUCK_23 C 470.32 3.8 4.497862 675.2930395BUCK_25 C 1066.92 4.7 16.710132 788.2029755BUCK_28 C 275.69 5.1 5.4497306 690.2842739BUCK_29 C 227.41 1.7 9.0337321 732.1180311BUCK_31 C 233.38 2.9 14.33371 773.6368641BUCK_33 C 510.91 18.2 3.0791918 647.0293044BUCK_34 C 103.09 1.5 6.2371579 701.1106577

AnalysisCores only


observed ±1s.e. 49Ti (ppm)

Table 4.5b: Zircon saturation thermometry of zircon xenocrystic cores from sample RNZ119 based on the method outlined in Watson et al. (2006).

Rims Ti-in-zircon only T (°C)

BUCK_02 R 110.98 0.5 5.4644938 690.4989309BUCK_04 R 135.25 1.1 4.9148744 682.15876BUCK_06 R 103.14 0.6 30.365347 849.0018538BUCK_07 R 537.44 14.2 3.3723152 653.6569557BUCK_10 R 101.45 0.5 4.1916011 669.902422BUCK_13 R 103.08 0.5 2.4635432 631.165597BUCK_15 R 108.83 0.6 6.8169702 708.375387BUCK_17 R 110.2 0.7 15.039803 778.1596404BUCK_19 R 107.22 0.9 15.538206 781.2483127BUCK_20 R 122.74 1.3 4.3148164 672.1096543BUCK_21 R 118.46 1.2 8.0890982 722.6684709BUCK_24 R 108.78 1 4.4831906 675.0419273BUCK_26 R 1080.67 4.6 12.282795 759.3728524BUCK_27 R 101.55 0.5 5.0985337 685.0287766BUCK_30 R 76.11 0.8 33.348063 859.1773898BUCK_32 R 300.95 3 14.670451 775.8160871BUCK_35 R 101.26 0.7 3.9495232 665.4024972

Analysis


observed ±1s.e. 49Ti (ppm)

Table 4.5c: Zircon saturation thermometry of zircon rims from sample RNZ119 based on the method outlined in Watson et al. (2006).

116

and are referred to as ‘hot temperature’ systems, with temperatures greater than 800˚C

(Miller et al. 2003, Hanchar and Watson 2003). Temperatures calculated using zircon

saturation thermometry are maximum and minimum estimates for the ‘cold’ and ‘hot’

temperature granitoids, respectively (Kemp et al. 2005a). The presence of both

inherited and igneous zircon in the Buckland Granite suggests the pluton was ‘cold’ in

temperature, under this classification.

Crystallisation temperatures of the Buckland Granite range from 697°C to 730°C,

which lies within the upper-amphibolite facies.

The ultramylonite outcropping along the coastline north of the Fox River mouth was

sampled at White Horse Creek (sample PCC06-02; Figure 2.7) for the isotopic dating

by Rb/Sr analysis of the fine-grained matrix, feldspar and muscovite grains present. A

Rb/Sr age of 116.2 ± 5.9 Ma with 2σ error was calculated for this sample, and is

proposed to represent a pre-ultramylonitic early stage of deformation (Figure 4.2;

Ring et al. 2006). The recrystallisation of feldspars within sample PCC06-02 indicates

that deformation was well within the amphibolite facies range at 116.2 ± 5.9 Ma –

further supporting the upper-amphibolite facies metamorphism of the Buckland

Granite during the Cretaceous.

4.1.4. Hafnium

Hafnium is a major element within zircon, with average HfO2 concentrations of 0.5-

2.0 wt% (Hoskin and Schaltegger 2003). The incompatible, highly immobile element

resists alteration due to its robust nature (Kemp et al. 2005b), and exhibits almost

identical geochemical behaviour as zircon (Claiborne et al. 2006), making it an

excellent tool for petrogenetic studies.

117

Figure 4.2: Plot of 87Sr/86Sr vs. 87Rb/86Sr of a mylonite (sample PCC06-02) from White Horse Creek at the southern end of the Paparoa Metamorphic Core Complex. A maximum age of ductile deformation is calculated as 116.2 ± 5.9 Ma (Ring et al. 2006).

Lutetium and hafnium are linked by the branched isobaric decay of 176Lu to 176Hf,

while 177Hf remains as a stable isotope (Dickin 2005, Belousova et al. 2006).

Corrections for in situ radiogenic growth are not required, due to the low

concentration of Lu-Hf ratios, and the high concentration of Hf within zircon (Yang et

al. 2007).

Lu-Hf isotopes provide the means to trace chemical changes within a body of rock

over time, with 176Hf/177Hf being calculated from the 176Lu/177Hf measured during

sampling (Griffin et al. 2000). Initial 176Hf/177Hf ratios represent the composition of

118

hafnium during the crystallisation of zircon, with the in situ decay of 176Lu considered

to be insignificant (Kinny and Maas 2003). The initial 176Hf/177Hf ratio remains

relatively unchanged during the evolution of a melt, allowing initial 176Hf/177Hf ratios

to be directly linked to crystallisation ages calculated from the in situ sampling of U-

Pb by laser ablation (Kinny and Maas 2003, Harrison et al. 2005).

Initial 176Hf/177Hf ratios of samples may be calculated using the Lu-Hf ratio, which

involves the use of the following formula, where t = elapsed time and λ = the 176Lu β-

decay constant (Blichert-Toft and Albarède 1997):

(176Hf/177Hf)t = (176Hf/177Hf)initial + (176Lu/177Hf)t · (eλt – 1)

Elapsed time since the crystallisation of the Buckland Granite from the Paparoa

Metamorphic Core Complex is calculated as 109.6 ± 1.7 Ma (Muir et al. 1992). The

value of the 176Lu β- decay constant is currently under revision, with 1.86 x 10-11 y -1

considered to be the most accurate estimate to date (Scherer et al. 2001, Kinny and

Maas 2003). Hf deviations from CHUR (chondritic uniform reservoir) are described

in parts per ten thousand as εHf (initial Hf isotope ratios) with the following formula,

where t = elapsed time (Kinny and Maas 2003):

εHf = [(176Hf/177Hf)t/(176Hf/177Hf)chondrites – 1] x 104

Any differences observed between measured 176Hf/177Hf values and calculated

(176Hf/177Hf)initial values are considered to be negligible.

εHf values may be either positive or negative with respect to chondritic176Hf/177Hf

(εHf = 0), with positive values (εHf > 0) indicating a mantle influence, while negative

119

values (εHf < 0) infer the involvement of crustal material within the igneous system

(Belousova et al. 2006, Hawkesworth and Kemp 2006).

Model ages may be calculated using Hf isotope ratios and 207Pb/206Pb ages, to

calculate the time at which magma bodies (zircon host rocks) were extracted from a

reference reservoir due to its robustness during alteration (Amelin et al. 1999, Nebel-

Jacobsen et al. 2005, Nebel et al. 2007). The use of 176Lu/177Hf and 176Hf/176Hf values

from Vervoort and Blichert-Toft (1999), along with U-Pb zircon ages allows a two-

stage model age (TDM) to be calculated. Reference reservoirs may either be the

depleted mantle (DM) or the chondritic uniform reservoir (CHUR). The isotopic

composition of a melt at the model age (TDM) can therefore be assumed to be the same

as the depleted mantle (Nebel et al. 2007). The depleted mantle is assumed to have an

average Hf isotope value (εHfDM) of 15.9, based on the values 0.038 and 0.283223 for

176Lu/177Hf and 176Hf/176Hf, respectively (Vervoort and Blichert-Toft 1999).

Felsic and mafic TDM values may be calculated for a magma to establish the

minimum and maximum timing of extraction from the mantle, respectively (Nebel et

al. 2007). Model ages allow the correlation of igneous rocks to major crustal-building

and/or orogenic events during the history of the Earth, with the Early Proterozoic to

Late Archean considered to be a period of accelerated crustal formation (Taylor and

McLennan 1985).

Hafnium concentrations were sampled by the in situ LA-ICP-MS method, for

samples RNZ119, UC08368, UC05592 and PCC06-01 from the Paparoa

Metamorphic Core Complex (Table 4.6a), with hafnium data for the standards used

being listed in Table 4.6b. Locations of analyses for each sample are shown in the

120

cathodoluminescence images (Figures 3.1a, 3.1b, 3.1c and 3.1d), which do not show

any evidence of cracking or inclusions that result in anomalous values.

εHf values have been calculated for zircons for sample RNZ119, following the

method outlined in Nebel and co workers (2007) in which U-Pb zircon ages are

correlated with Hf values of individual zircon grains.

Calculated εHf values (including maximum and minimum εHf) and TDM of all

analyses are listed in Table 4.7. εHf values ranged from 14.4 to -22.2, with an average

value of -1.7 and a standard deviation of 8.0. Maximum εHf (uncorrected) values have

an average of 1.8, with a standard deviation of 8.2. Minimum εHf (uncorrected) values

have an average of -4.7, with a standard deviation of 7.8.

TDM mafic and TDM felsic values for sample RNZ119 average at 3410 Ma and 2969

Ma, respectively, with the mafic TDM value of 3410 Ma coinciding with the proposed

major crustal formation event of the Gondwana supercontinent at c. 3.4-3.5 Ga (Kemp

et al. 2006).

εHf values were also calculated separately for zircon xenocrystic cores and rims for the

sample RNZ119 (Tables 4.8a and 4.8b). Xenocrystic cores had an average εHf value of

-0.1, with a standard deviation of 11.1. Maximum εHf (uncorrected) values for

xenocrystic cores have an average of 3.3, with a standard deviation of 2.3. Minimum

εHf (uncorrected) values for xenocrystic cores have an average of -3.0, with a standard

deviation of 1.7. Zircon rims had an average εHf value of 0.0, with a standard

deviation of 13.5.

121

Analysis C vs. R Volts 176Hf/177Hf 2σ error 176Lu/177Hf corrected 176Lu/177Hf

RNZ119HfB_01 R 1.11 0.282997 0.000308 0.00239 0.00227HfB_02 C 1.9 0.282671 0.000088 0.00025 0.00024HfB_03 C 2.12 0.282339 0.000074 0.0005 0.00047HfB_04 R 3.44 0.282702 0.000067 0.00119 0.00113HfB_05 C 2.16 0.281748 0.000093 0.00082 0.00078HfB_06 R 2.21 0.281804 0.00009 0.0009 0.00085HfB_07 C 2.36 0.282711 0.000088 0.00031 0.0003HfB_08 R 1.87 0.282837 0.000157 0.00106 0.00101HfB_09 C 1.7 0.282804 0.0001 0.00055 0.00053HfB_10 R 4.44 0.282785 0.000074 0.00169 0.00161

UC08368Hf68_01 C 2.44 0.282532 0.000077 0.00092 0.00088Hf68_02 C 1.63 0.282164 0.000093 0.00051 0.00049Hf68_03 R 2.43 0.28252 0.000074 0.00049 0.00047Hf68_04 C 1.73 0.282423 0.0001 0.00148 0.00141Hf68_05 R 2.26 0.282504 0.000083 0.00152 0.00145Hf68_06 C 1.78 0.282082 0.000101 0.00042 0.0004Hf68_07 R 1.4 0.282136 0.000105 0.00088 0.00084

UC05592Hf92_01 C 1.79 0.282646 0.000095 0.00197 0.00188Hf92_02 R 1.99 0.282828 0.000092 0.0021 0.002Hf92_03 R 2.91 0.282545 0.000073 0.0002 0.00019Hf92_04 R 2.41 0.282369 0.000085 0.00105 0.001Hf92_05 R 3.08 0.282843 0.000101 0.00276 0.00263Hf92_06 C 1.33 0.283096 0.000249 0.00153 0.00146

PCC06-01HfS_01 R 1.9 0.282167 0.000097 0.00064 0.00061HfS_02 C 1.41 0.282135 0.000123 0.00137 0.0013HfS_03 C 1.74 0.281889 0.000094 0.00089 0.00085HfS_04 R 2.36 0.281901 0.000085 0.00106 0.00101HfS_05 R 1.71 0.282072 0.000088 0.00117 0.00112HfS_06 C 1.42 0.282145 0.000108 0.00127 0.00121HfS_07 C 1.51 0.282482 0.000116 0.00048 0.00046HfS_08 R 2.42 0.282525 0.000076 0.00048 0.00046

Table 4.6a: Hafnium data for samples RNZ119, UC08368, UC5592 and PCC06-01. Alternating font style (bold vs. regular) shows analyses common to individual zircon grains. Analyses in italics are samples spots which are not common to individual grains, and are not included in the Hf calculations.

Analysis Volts 176Hf/177Hf 2σ error 176Lu/177Hf measured-accepted 266_01 1.05 0.281646 0.000125 0.000222 1.60E-05266_02 1.06 0.281594 0.000142 0.000219 -3.60E-05266_03 1.07 0.281613 0.000152 0.000219 -1.70E-05

Table 4.6b: Hafnium data for standards used for the samples UC08368, UC5592 and PCC06-01.

122

Analysis C vs. R 176Hf/177Hf 2σ errorLu/Hf

(corrected) U-Pb age (Ma) ± 1 seHf age

(corrected)Hf CHUR

at T

HfB_01 R 0.282997 0.000308 0.00227 314.6349783 17.57113524 0.282984 0.282577HfB_02 C 0.282671 0.000088 0.00024 116.3241816 1.712684739 0.28267 0.2827HfB_03 C 0.282339 0.000074 0.00047 636.0121967 5.858756696 0.282333 0.282376HfB_04 R 0.282702 0.000067 0.00113 101.3314408 0.479427994 0.2827 0.282709HfB_05 C 0.281748 0.000093 0.00078 1604.237201 29.27925056 0.28174 0.282422HfB_06 R 0.281804 0.00009 0.00085 562.5694685 14.85492491 0.281778 0.281763HfB_07 C 0.282711 0.000088 0.0003 102.7025288 0.415364854 0.28271 0.282708HfB_08 R 0.282837 0.000157 0.00101 109.3607797 0.63347052 0.282835 0.282704HfB_09 C 0.282804 0.0001 0.00053 112.5138063 0.718688839 0.282803 0.282702HfB_10 R 0.282785 0.000074 0.00161 135.4818137 1.05858967 0.282781 0.282688Average - 0.2825398 0.0001139 0.000919 379.5168395 7.2582294 0.2825398 0.0001139

Analysis C vs. R εHfεHf max

(uncorrected)εHf min

(uncorrected) HfDM at T calculated TDM TDM mafic TDM felsic

HfB_01 R 14.4 25.8 4 0.282999 344 3410 2969HfB_02 C -1 2.1 -4.1 0.28314 987 3410 2969HfB_03 C -1.5 1.3 -3.9 0.282769 1444 3410 2969HfB_04 R -0.3 2.1 -2.6 0.283151 937 3410 2969HfB_05 C -1.4 2.8 -3.8 0.282068 2243 3410 2969HfB_06 R -22.2 -18.7 -25.1 0.282822 2447 3410 2969HfB_07 C 0.1 3.2 -3 0.28315 918 3410 2969HfB_08 R 4.6 10.3 -0.9 0.283145 686 3410 2969HfB_09 C 3.6 7.1 0.1 0.283143 744 3410 2969HfB_10 R 3.3 6.1 0.8 0.283127 778 3410 2969Average - -1.7 1.8 -4.7 0.282952 1243 3410 2969Std dev - 8 8.2 7.8 0 664.3 - -

Table 4.7: Combined LA-ICP-MS data for sample RNZ119, with calculated values for εHf and TDM for all analyses. The calculation of εHf follows the method used by Nebel et al. (2007), with U-Pb ages sourced from individual zircon grains. Averages and standard deviations are shown for εHf, εHf max (uncorrected), εHf min (uncorrected), HfDM at T and calculated TDM.

123

Analysis C vs. R 176Hf/177Hf 2σ error Lu/HfU-Pb age

(Ma) ± 1 se Hf ageHf CHUR

at T

HfB_02 C 0.282671 0.000088 0.00024 116.32418 1.7126847 0.28267 0.2827HfB_03 C 0.282339 0.000074 0.00047 636.0122 5.8587567 0.282333 0.282376HfB_05 C 0.281748 0.000093 0.00078 562.56947 14.854925 0.28174 0.282422HfB_07 C 0.282711 0.000088 0.0003 102.70253 0.4153649 0.28271 0.282708HfB_09 C 0.282804 0.0001 0.00053 112.51381 0.7186888 0.282803 0.282702Average - 0.2824546 8.86E-05 0.000464 306.02444 4.712084 0.2824546 8.86E-05

Analysis C vs. R εHf εHf max εHf min HfDM at Tcalculated

TDM TDM mafic TDM felsic

HfB_02 C -1 2.1 -4.1 0.28314 987 3410 2969HfB_03 C -1.5 1.3 -3.9 0.282769 1444 3410 2969HfB_05 C -24.1 -20.6 -27.1 0.282822 2546 3410 2969HfB_07 C 0.1 3.2 -3 0.28315 918 3410 2969HfB_09 C 3.6 7.1 0.1 0.283143 744 3410 2969Average - -4.6 -1.4 -7.6 0.283005 1328 3410 2969Std dev - 11.1 11 11 0 728.7 - -

Table 4.8a: Combined LA-ICP-MS data for sample RNZ119, with calculated values for εHf and TDM for zircon xenocrystic cores only. The calculation of εHf follows the method used by Nebel et al. (2007), with U-Pb ages sourced from individual zircon grains. Averages and standard deviations are shown for εHf, εHf max (uncorrected), εHf min (uncorrected), HfDM at T and calculated TDM.

Analysis C vs. R 176Hf/177Hf 2σ error Lu/HfU-Pb age

(Ma) ± 1 se Hf ageHf CHUR

at T

HfB_01 R 0.282997 0.000308 0.00227 314.63498 17.571135 0.282984 0.282577HfB_04 R 0.282702 0.000067 0.00113 101.33144 0.479428 0.2827 0.282709HfB_06 R 0.281804 0.00009 0.00085 1604.2372 29.279251 0.281778 0.281763HfB_08 R 0.282837 0.000157 0.00101 109.36078 0.6334705 0.282835 0.282704HfB_10 R 0.282785 0.000074 0.00161 135.48181 1.0585897 0.282781 0.282688Average - 0.282625 0.000139 0.001374 453.00924 9.8043748 0.282625 0.000139

Analysis C vs. R εHf εHf max εHf min HfDM at Tcalculated

TDM TDM mafic TDM felsic

HfB_01 R 14.4 25.8 4 0.282999 344 3410 2969HfB_04 R -0.3 2.1 -2.6 0.283151 937 3410 2969HfB_06 R 0.5 4.6 -1.7 0.282068 2143 3410 2969HfB_08 R 4.6 10.3 -0.9 0.283145 686 3410 2969HfB_10 R 3.3 6.1 0.8 0.283127 778 3410 2969Average - 4.5 9.8 -0.1 0.282898 978 3410 2969Std dev - 5.9 9.4 2.6 0 687 - -

Table 4.8b: Combined LA-ICP-MS data for sample RNZ119, with calculated values for εHf and TDM for zircon rims only. The calculation of εHf follows the method used by Nebel et al. (2007), with U-Pb ages sourced from individual zircon grains. Averages and standard deviations are shown for εHf, εHf max (uncorrected), εHf min (uncorrected), HfDM at T and calculated TDM.

124

Maximum εHf (uncorrected) values for zircon rims have an average of 5.1, with a

standard deviation of 16.0. Minimum εHf (uncorrected) values for zircon rims have an

average of -4.7, with a standard deviation of 11.6.

εHf values of zircons from sample RNZ119 are both negative and positive, with an

average negative value of -1.7. The presence of negative εHf values indicates crustal

involvement during the crystallisation of zircon, as expected for the S-type nature of

the Buckland Granite.

4.1.5. REE’s

The REE’s are a group of trivalent cation elements, which successfully record

geochemical processes taking place during the petrogenesis of igneous and

metamorphic rocks (Wark and Miller 1993, Cherniak and Watson 2003). Elements

commonly included within the REE group are yttrium, lanthanum and the lanthanides,

from cerium to lutetium (Henderson 1996), which is adopted for this thesis.

REE’s are considered to be incompatible in silicate magmatic systems, and reside in

the melt during crystallisation of magma. REE’s become incorporated into minerals

with larger cations, including zircon, titanite, apatite, epidote and garnet (Henderson

1996). REE’s are subdivided into the light REE’s (LREE’s), which range from

lanthanum to samarium, and the heavy REE’s (HREE’s), from gadolinium to lutetium

(Henderson 1996).

Zircon is an excellent tool in geochemical investigations into the petrogenesis of

igneous and metamorphic rocks, and is commonly sampled by in situ techniques

including SIMS and LA-ICP-MS. Unaltered zircons characteristically show a

reduction in LREE’s and an increase in HREE’s, with positive Ce and negative Eu

anomalies (Hoskin and Schaltegger 2003). Zircons which have not suffered alteration

125

generally have very low concentrations of L, Be, B, F, Na, Mg, Al, P, Ca, Sc, Ti, V,

Cr, Mn, Fe, Sr, Nb, Ba and Ta (Hoskin and Schaltegger 2003). Zircons which have

experienced alteration often display a relative increase in LREE’s and reduction in

HREE’s, through the leaching of the highly mobile elements during hydrothermal

fluid movement through the crust (Tulloch and Christie 2000). The role of fluid flow

on the behaviour of REE’s in the continental crust is of importance, as REE’s are

considered mobile in aqueous systems. Hydrothermal fluids encourage the leaching

and transport of REE’s along faults or fractures in the upper crust (Henderson 1996).

Previous geochemical work has been undertaken in the Paparoa Metamorphic Core

Complex, with the sampling of REE concentrations in rocks adjacent to the Ohika and

Pike Detachment Faults of the Paparoa Metamorphic Core Complex (Figure 4.3a;

Christie et al. 1998, Tulloch and Christie 2000). Depleted concentrations of REE’s

were observed in granitoid rocks of the Lower Plate, while elevated concentrations

were found in the Greenland Group metasediments and Hawks Crag Breccia units of

the Upper Plate. Depletions and elevations of REE concentrations are relative to

‘background rocks’, which include geochemical data from unaltered samples of the

Buckland and Charleston Granites (Tulloch and Christie 2000). The reduction of

REE’s in Lower Plate rocks is proposed to have taken place through the leaching of

elements by hydrothermal fluids during the rapid exhumation of the Lower Plate

granitoid rocks during extension. Fluids are suggested to have transported the mobile

REE’s into the Upper Plate, to become concentrated within the water-saturated cover

rocks of the metamorphic core complex (Tulloch 1995). This REE analysis showed a

negative trend with an enrichment of LREE’s and depletion of HREE’s (Figure 4.3b;

126

Figure 4.3a: Plot of SiO2 vs. REE’s of rocks adjacent to the Pike and Ohika

Detachment Faults of the Paparoa Metamorphic Core Complex (Tulloch and Christie 2000).

Figure 4.3b: Plot of chondrite-normalised REE’s of unaltered granitoids of the Paparoa Metamorphic Core Complex (Tulloch and Christie 2000).

127

Tulloch and Christie 2000). It is important to note that the proposed role of

hydrothermal fluids on REE concentrations within the Lower Buller Gorge area is

based on very few samples, and may only reflect a localised geochemical process

(Tulloch and Christie 2000).

Further geochemical investigations have been carried out, with the sampling of

REE’s of rocks associated with both the Ohika and Pike Detachment Faults of the

Paparoa Metamorphic Core Complex. The concentrations of REE’s 139La, 140Ce,

147Sm, 153Eu, 163Dy and 175Lu, along with 89Y, were measured during the in situ LA-

ICP-MS analyses of zircons from samples UC05592, UC08368, RNZ119 and PCC06-

01 (Tables 4.9a, 4.9b, 4.9c and 4.9d). Remaining REE’s were interpolated from these

values, and plotted in chondrite-normalised REE diagrams. Xenocrystic cores and

rims are plotted separately, to allow comparisons to be made both within and between

samples. The break of trends in the following chondrite-normalised REE diagrams

exists due to the exclusion of Pm values.

Ce and Eu anomalies within the data followed the expected positive and negative

trends, respectively for unaltered igneous zircon. The presence of anomalously high

levels of P and La in some analyses is considered to represent the sampling of mineral

inclusions, such as monazite and apatite, within individual zircon grains. These

analyses are therefore considered to be outliers, and have not been included in the

chondrite-normalised REE diagrams.

UC05592

The REE concentrations of xenocrystic zircon cores are shown in Figure 4.4a, with

analyses 92-03, 92-05, 92-08 and 92-11. REE patterns of zircon rims are plotted

128

129

130

131

132

133

134

135

sample UC05592 (cores only)

0.10

1.00

10.00

100.00

1000.00

10000.00

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Rare Earth Elements

chon

drite

-nor

mal

ised

(ppm

)92_0392_0592_0892_11

Figure 4.4a: REE chondrite-normalised diagram showing values of zircon xenocrystic cores from sample UC05592.

separately in Figure 4.4b, with analyses 92-06, 92-07- 92-10 and 92-12. Element

concentrations for each analysis are listed in Table 4.9a. Analyses 92-09 and 92-13

were considered to be outliers, and were removed from the data set. Analysis 92-09

exhibited anomalously high levels of both P and Lu, which is suggested to represent

the sampling of a mineral inclusion present within the zircon grain. Analysis 92-13

was removed due to its variation from the general positive trend within the data set.

Distinctive anomalies are observed in both the ‘cores only’ and ‘rims only’ plots,

with Ce and Eu having positive and negative anomalies, respectively.

The average REE values of xenocrystic cores and rims are plotted together, with

obvious positive and negative anomalies for Ce and Eu, respectively (Figure 4.4c).

136

sample UC05592 (rims only)

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(p

pm)

92_0692_0792_1092_12

Figure 4.4b: REE chondrite-normalised diagram showing values of zircon rims from sample UC05592.

sample UC05592 (cores vs. rims)

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(p

pm)

cores only (average)rims only (average)

Figure 4.4c: REE chondrite-normalised diagram showing average REE values of xenocrystic cores and rims of zircons from sample UC05592.

137

UC08368

The REE concentrations of xenocrystic zircon cores are shown in Figure 4.4d, with

analyses 68-01, 68-03, 68-05, 68-09, 68-11, 68-13, 68-19 and 68-21. REE patterns of

zircon rims are plotted separately in Figure 4.4e, with analyses 68-02, 68-04, 68-07,

68-08, 68-10, 68-20 and 68-22. Element concentrations for each analysis are listed in

Table 4.9b.

Analyses 68-06, 68-12 and 68-18 were considered to be outliers, and were removed

from the data set. Analysis 68-06 exhibited anomalously high levels of both P and Lu,

which is suggested to represent the sampling of a mineral inclusion present within the

zircon grain. Analyses 68-12 and 68-18 were removed due to their variation from the

general positive trend within the data set.



The average REE values of xenocrystic cores and rims are plotted together, with an

obvious negative Eu anomaly (Figure 4.4f). No trend is shown for elements La to Nd,

and therefore no anomaly is shown for Ce, due to the limited La values required for

the interpolation of Pr and Nd values.

RNZ119

The REE concentrations of xenocrystic zircon cores are shown in Figure 4.4g, with

analyses BUCK-01, BUCK-03, BUCK-05, BUCK-08, BUCK-09, BUCK-11, BUCK-

12, BUCK-14, BUCK-16, BUCK-18, BUCK-22, BUCK-23, BUCK-25, BUCK-28,

BUCK-29 and BUCK-34. REE patterns of zircon rims are plotted separately in Figure

4.4h, with analyses BUCK-02, BUCK-04, BUCK-06, BUCK-07, BUCK-13, BUCK-

138

sample UC08368 (cores only)

0.10

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(ppm

)68_0168_0368_0568_0968_1168_1368_1968_21

Figure 4.4d: REE chondrite-normalised diagram showing values of zircon xenocrystic cores from sample UC08368.

sample UC08368 (rims only)

0.10

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(ppm

)

68_0268_0468_0768_0868_1068_2068_22

Figure 4.4e: REE chondrite-normalised diagram showing values of zircon rims from sample UC08368.

139

sample UC08368 (cores vs. rims)

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(ppm

)


Figure 4.4f: REE chondrite-normalised diagram showing average REE values of xenocrystic cores and rims of zircons from sample UC08368.

sample RNZ119 (cores only)

0.01

0.10

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(p

pm)

BUCK_01BUCK_03BUCK_05BUCK_08BUCK_09BUCK_11BUCK_12BUCK_14BUCK_16BUCK_18BUCK_22BUCK_23BUCK_25BUCK_28BUCK_29BUCK_34

Figure 4.4g: REE chondrite-normalised diagram showing values of zircon xenocrystic cores from sample RNZ119.

140

sample RNZ119 (rims only)

0.10

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(ppm

)

BUCK_02BUCK_04BUCK_06BUCK_07BUCK_13BUCK_15BUCK_17BUCK_20BUCK_21BUCK_24BUCK_26BUCK_27BUCK_35

Figure 4.4h: REE chondrite-normalised diagram showing values of zircon rims from sample RNZ119.

15, BUCK-17, BUCK-20, BUCK-21, BUCK-24, BUCK-26, BUCK-27 and BUCK-

35. Element concentrations for each analysis are listed in Table 4.9c.

Analyses BUCK-10, BUCK-19, BUCK-30, BUCK-31, BUCK-32 and BUCK-33

were considered to be outliers, and were removed from the data set. Analyses BUCK-

19, BUCK-30 and BUCK-32 exhibited anomalously high levels of both P and Lu,

which is suggested to represent the sampling of a mineral inclusion present within the

zircon grain. Analyses BUCK-10, BUCK-31 and BUCK-33 were removed due to

their variation from the general positive trend within the data set.



141


obvious negative Eu anomaly (Figure 4.4i). No trend is shown for elements La to Nd,



PCC06-01

The REE concentrations of xenocrystic zircon cores are shown in Figure 4.4j, with

analyses S-01, S-03, S-05, S-10, S-12, S-16, S-19, S-21, S-22, S-24, S-26, S-28, S-30

and S-34. REE patterns of zircon rims are plotted separately in Figure 4.4k, with

analyses S-02, S-04, S-06, S-08, S-09, S-11, S-13, S-15, S-17, S-20, S-23, S-25, S-27,

S-29, S-31, S-33 and S-35. Element concentrations for each analysis are listed in

Table 4.9d.

Analyses S-18 and S-32 were considered to be outliers, and were removed from the

data set. Analysis S-18 exhibited anomalously high levels of both P and Lu, which is

suggested to represent the sampling of a mineral inclusion present within the zircon

grain. Analysis S-32 was removed due to its variation from the general positive trend

within the data set.




obvious negative Eu anomaly (Figure 4.4l). No trend is shown for elements La to Nd,



142

sample RNZ119 (cores vs. rims)

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(ppm

)


Figure 4.4i: REE chondrite-normalised diagram showing average REE values of xenocrystic cores and rims of zircons from sample RNZ119.

sample PCC06-01 (cores only)

0.01

0.10

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(ppm

)

S_01S_03S_05S_10S_12S_16S_19S_21S_22S_24S_26S_28S_30S_34

Figure 4.4j: REE chondrite-normalised diagram showing values of zircon xenocrystic cores from sample PCC06-01.

143

sample PCC06-01 (rims only)

0.01

0.10

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(ppm

)S_02S_04S_06S_07S_08S_09S_11S_13S_15S_17S_20S_23S_25S_27S_29S_31S_33S_35

Figure 4.4k: REE chondrite-normalised diagram showing values of zircon rims from sample PCC06-01.

sample PCC06-01 (cores vs. rims)

1.00

10.00

100.00

1000.00

10000.00


Rare Earth Elements

chon

drite

-nor

mal

ised

(ppm

)


Figure 4.4l: REE chondrite-normalised diagram showing average REE values of xenocrystic cores and rims of zircons from sample PCC06-01.

144

The REE patterns for UC05592, UC08368, RNZ119 and PCC06-01 all show

positive trends with a relative enrichment of HREE’s and depletion of LREE’s, as

expected for zircon-bearing rocks which have not suffered extensive alteration. These

findings contrast with those of Tulloch and Christie (2000), where alteration led to the

(at least) localised leaching of the HREE’s of Lower Plate rocks adjacent to the Ohika

Detachment Fault in the Lower Buller Gorge area, to produce the negative slope in the

chondrite-normalised REE diagram (Figure 4.3b).

Sample PCC06-01, sourced just north of the inferred Pike Detachment Fault, does

not display any evidence for the leaching of REE’s by hydrothermal fluid movement

along the fault plane during extension.

4.2. Geochronological investigations

The use of geochronology in igneous and metamorphic petrogenetic studies provides

information on the timing of crystallisation and metamorphism within an individual

rock unit. In situ sampling of U-Th-Pb concentrations in individual zircon grains

allows U-Pb dates to be calculated, with greater accuracy than using the whole rock

approach.

4.2.1. U-Pb dating

The U-Th-Pb system involves the decay of radioactive U isotopes to radiogenic Pb

isotopes, through the following sequences (Winter 2001).

238U → 234U → 206Pb (λ = 1.5512 × 10 – 10 a – 1)

238U → 235U → 207Pb (λ = 9.8485 × 10 – 10 a – 1)

238U → 232Th → 208Pb (λ = 4.9475 × 10 – 11 a – 1)

145

U, Th and Pb concentrations are recorded in zircons, which may either crystallise

from the primary melt, or become incorporated into the igneous system through

crustal contamination. Subsequent metamorphic events are also recorded, through the

growth of metamorphic rims around the older zircon crystals.

The ratio of Th and U content within zircon may be used to determine whether

zircon growth is magmatic or metamorphic in nature, with age corrections only

required for samples with huge variations in U-Pb ages (Maas et al. 1992). Typical

igneous zircons have been found to record Th/U ratios ranging from 0.2 to 1.0, while

metamorphic zircons generally show much smaller Th/U ratios ranging from 0.1 to

<0.05 (Williams and Claesson 1987, Schiøtte et al. 1988a, Schiøtte et al. 1988b,

Schiøtte et al. 1989, Kinny et al. 1990).

The in situ sampling of U and Pb in zircon crystals by LA-ICP-MS allows the dating

of petrogenetic events in igneous and metamorphic rocks, using the U-Pb dating

method. The behaviour of U and Pb may be visually represented in Tera-Wasserburg

diagrams, where 207Pb/206Pb is plotted against 238U/206Pb (Tera and Wasserburg 1972).

Tera-Wasserburg diagrams were constructed using the Isoplot Ex v.2.6. program

(Ludwig 1999). The alignment of data into an array allows the distinction to be made

between common-Pb (207Pb/206Pb at the upper intercept) and radiogenic Pb

(206Pb*/238U at the lower intercept) (Parrish and Noble 2003).

The presence of an array in Tera-Wasserburg diagrams reflects the concordant nature

of the data, as the 207Pb*/235U and 206Pb*/238U ages are consistent with each other.

Concordant data is considered reliable, as it records the various stages of zircon

growth and recrystallisation through time, without any evidence of significant

alteration.

146

Data points which do not display a linear relationship when plotted in Tera-

Wasserburg diagrams suggest discordance within the data set. Discordance within

geochemical data may develop through extracrystalline diffusion, through the removal

and addition of elements such as U, Th and Pb during alteration. U-Pb ages become

affected by this style of diffusion, as the removal of such elements does not involve

radioactive decay, and Pb isotopes become depleted with respect to their

concentration in the rock (Wayne and Sinha 1988, Ashwal et al. 1999, Winter 2001).

It is due to this decrease in Pb isotopes that discordant ages may be attributed to Pb-

loss through diffusion by some authors (Cherniak and Watson 2003, Parrish and

Noble 2003), however, the slow diffusion rates of Pb in crystalline zircon makes Pb-

loss by volume diffusion unlikely (Cherniak and Watson 2000). Pb-loss by diffusion

is more likely to develop in metamict zircon grains that have experienced damage to

the crystal lattice associated with U and Th radioactive decay, allowing the

recrystallisation and/or mobilisation of Pb to occur (Silver 1963, Pidgeon et al. 1968,

Whitehouse et al. 1999, Cherniak and Watson 2000, Bowring and Schmitz 2003).

Connelly (2000) discusses the relationship between intracrystalline deformation and

the degree of preservation of primary growth zoning within individual igneous zircon

grains. Connelly suggests that variations in the preservation of growth zoning may be

used as a proxy for the amount of intracrystalline diffusion that has taken place within

individual grains. Intracrystalline diffusion and recrystallisation of zircon results in

concordance within the geochemical data, as changes to the chemistry of individual

grains takes place within closed systems.

Sampling of sections with poorly preserved growth zoning patterns have been shown

to produce discordant data, however, this relationship must not be assumed to be the

147

case for all zircons. CL imaging prior to geochemical sampling clearly becomes an

invaluable tool, as any grains that appear to have experienced alteration may be

avoided to reduce chances of discordance within samples (Connelly 2000).

The four samples from the Paparoa Metamorphic Core Complex (UC05592,

UC08368, PCC06-01 and RNZ119) were analysed with the LA-ICP-MS technique,

for the concentrations of U, Th and Pb (Tables 4.9a, 4.9b, 4.9c and 4.9d).

Tera-Wasserburg concordia diagrams and probability density distribution-histogram

plots were constructed to visually represent U-Pb behaviour for each sample. The

imaging of zircon grains from each sample by CL allowed any anomalous features to

be avoided during sampling (Figures 3.1a, 3.1b, 3.1c and 3.1d). Zircon analysis

involved both xenocrystic core and rim sampling, in attempt to illustrate chemical

variations between inherited and metamorphic crystallisation histories.

UC05592

The probability density distribution-histogram plot for UC05592, with all analyses

included, shows inheritance of zircon grains up to c. 1000 Ma old. Two main peaks

are noted at c. 300 and c. 100 Ma (Figure 4.5a). Data points around c. 300 Ma are

illustrated in a separate probability density distribution-histogram plot, which shows

three main peaks within the limited data set at c. 250, c. 300 and c. 330 Ma (Figure

4.5b). Data plotted in the Tera-Wasserburg concordia diagram do not display a strong

linear relationship, and are considered to be discordant (Figure 4.5c).

148

0

1

2

3

4

5

6

7

0 200 400 600 800 1000 1200

Age (Ma)

Num

ber

Relative probability

Figure 4.5a: Probability density distribution-histogram plot of all data for sample UC05592.

0

1

2

200 220 240 260 280 300 320 340 360 380

Age (Ma)

Num

ber


Figure 4.5b: Probability density distribution-histogram plot of inherited c. 300 Ma zircons for sample UC05592.

149

200600

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 20 40 60 80

238U/206Pb

207 Pb

/206 Pb

Figure 4.5c: Tera-Wasserburg concordia diagram of sample UC05592. Data-point error ellipses are 2σ.

UC08368

The probability density distribution-histogram plot for UC08368, with all analyses

included, has inherited zircon up to c. 1400 Ma in age. Two main peaks exist at c.

1000 and c. 300 Ma (Figure 4.6a). The probability density distribution-histogram plot

for UC08368, with the exclusion of inherited grains, ranges between c. 410 and 330

Ma with three peaks at c. 400, c. 360 and c. 335 Ma (Figure 4.6b). Data plotted in the

Tera-Wasserburg concordia diagram follows a linear relationship, with low error

values. The data is therefore considered to be concordant in nature (Figure 4.6c).

150

0

1

2

3

4

5

6

7

8

9

10

200 400 600 800 1000 1200 1400 1600

Num

ber


Age (Ma)

Figure 4.6a: Probability density distribution-histogram plot of all data for sample UC08368.

0

1

2

320 340 360 380 400 420 440

Num

ber


Age (Ma)

Figure 4.6b: Probability density distribution-histogram plot of inherited c. 300 to c. 400 Ma zircons for sample UC08368.

151

2000

1600

1200

800

400

0.04

0.06

0.08

0.10

0.12

0.14

0 4 8 12 16 20 24 238U/206Pb

206 Pb

/207 P

b

Figure 4.6c: Tera-Wasserburg concordia diagram of sample UC08368. Data-point error ellipses are 2σ.

PCC06-01

The probability density distribution-histogram plot for PCC06-01, with all analyses

included, shows the presence of up to c. 2000 Ma inherited zircon within the

mylonite. Two main peaks are present within this data set, at c. 600 and c. 300 Ma

(Figure 4.7a). The probability density distribution-histogram plot for PCC06-01, with

the exclusion of inherited grains, ranges from c. 700 to c. 300 Ma, with two main

peaks at c. 550 and c. 350 Ma (Figure 4.7b).

Data plotted in the Tera-Wasserburg concordia diagram follows a linear relationship,

with the data considered to be concordant in nature (Figure 4.7c).

152

0

1

2

3

4

5

6

7

200 600 1000 1400 1800 2200

Age (Ma)

Num

ber


Figure 4.7a: Probability density distribution-histogram plot of all data for sample PCC06-01.

0

1

2

3

4

5

300 400 500 600 700 800

Age (Ma)

Num

ber


Figure 4.7b: Probability density distribution-histogram plot of data <700 Ma for sample PCC06-01.

153

600

1000

1400

1800

2200

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 4 8 12 16 20 24 238U/206Pb

206 Pb

/207 Pb

Figure 4.7c: Tera-Wasserburg concordia diagram of all data from sample PCC06-01. Data-point error ellipses are 2σ. RNZ119

U-Pb dating of the Buckland Granite has been carried out by Muir and co workers

(1994), through the in situ analysis of zircon grains using the Sensitive High

Resolution Ion Micro Probe (SHRIMP) technique. Concordant U/Pb ages from Muir

and co workers (1994) are listed in Table 4.10, and have been plotted in probability

density distribution-histogram diagrams. All SHRIMP data points are plotted in

Figure 4.8a, with the inheritance of zircon up to c. 1000 Ma and a main peak at c. 110

Ma. The probability density distribution-histogram plot of SHRIMP data, with the

exclusion of inherited grains, records two main peaks at c. 118 and c. 109 Ma (Figure

4.8b).

154

Concordant 238U/206Pb Age ±1se Concordant 238U/206Pb Age ±1se

120 2.6 105.1 2.5492.9 10.9 880.3 19.8117.8 2.6 107.9 2.5538.1 11.9 117.1 3.2463.8 10.2 116.1 2.7111.8 2.5 110.9 2.4112.5 2.5 105.5 2.8824.2 18.2 112.1 2.9106.8 2.4 106.8 2.8559.8 12.4 924.9 23.5690 15.2 117.6 3

299.7 6.6 492.1 12.4107.9 2.5 117.8 3360.8 7.9 113.5 2.9599 13.4 114 3.2

645.3 14.3 111.2 3

Table 4.10: SHRIMP data for the Buckland Granite (sample RNZ119) by Muir et al. (1994).

0

5

10

15

20

0 200 400 600 800 1000 1200

Num

ber


Age (Ma)

Figure 4.8a: Probability density distribution-histogram plot of all SHRIMP data for sample RNZ119 (Muir et al. 1994).

155

0

1

2

3

4

5

6

95 100 105 110 115 120 125 130 135

Num

ber


Age (Ma)

Figure 4.8b: Probability density distribution-histogram plot of SHRIMP data excluding inherited zircons for sample RNZ119 (Muir et al. 1994).

The analysis of RNZ119 was carried out using the LA-ICP-MS technique for this

thesis, with the in situ systematic sampling of both zircon xenocrystic cores and rims

(Table 3.2a). The probability density distribution-histogram plot for RNZ119, with all

analyses included, shows the incorporation of zircon up to c. 1600 Ma, with a peak at

c. 100 Ma (Figure 4.9a). The probability density distribution-histogram plot for

RNZ119, with the exclusion of inherited grains, reflects two main peaks at 102.4 ±

0.7 and 110.3 ± 0.9 Ma (Figure 4.9b).

Data plotted in the Tera-Wasserburg concordia diagram highlights the two distinct

age peaks at 102.4 ± 0.7 and 110.3 ± 0.9 Ma, with the data considered to be

concordant due to the distinct linear relationship (Figure 4.9c).

156

0

5

10

15

20

25

0 400 800 1200 1600 2000

Age (Ma)

Num

ber


Figure 4.9a: Probability density distribution-histogram plot of all data for sample RNZ119.

0

1

2

3

4

94 98 102 106 110 114 118 122

Age (Ma)

Num

ber R

elative probability

Figure 4.9b: Probability density distribution-histogram plot excluding inherited zircons for sample RNZ119.

157

118114 110

106 1020.04

0.06

0.08

0.10

54 56 58 60 62 64

238U/206Pb

207 Pb

/206 Pb

110.3±0.9 Ma(MSDW=0.4, N=9)

102.4±0.7 Ma(MSDW=0.2, N=6)

Figure 4.9c: Tera-Wasserburg concordia diagram of all data from sample PCC06-01. Data-point error ellipses are 2σ.

4.3. Combined geochemistry and geochronology

The combining of geochemical and geochronological data allows the observation of

chemical changes during the evolution of a melt to be made, through the tracking of

changes from zircon cores to rims in individual grains. The behaviour of O, Hf

isotope ratios and Ti concentrations during crystallisation is constrained by U-Pb

zircon ages, allowing petrogenetic interpretations to be made.

Zircons from the Buckland Granite are investigated in this section, due to the

presence of igneous zircons which crystallised at the timing of emplacement during

regional-scale extension in the Cretaceous.

Some of the following diagrams used to reflect changes during the evolution of the

Buckland Granite have arrows linking individual zircon xenocrystic cores and rims

158

where appropriate. Diagrams tracking oxygen isotope behaviour have the average

δ18O mantle value of 5.3 ± 0.3‰ represented by the stippled area, with values greater

than 5.3 ± 0.3‰ indicative of the incorporation of a sedimentary component into the

melt (Eiler 2001, Valley 2003, Valley et al. 2005, Kemp et al. 2006, Kemp et al.

2007). δ18O values greater than 5.3 ± 0.3‰ are consistent with S-type granites, which

develop through the incorporation of crustal material into a once primitive magma.

The behaviour of oxygen isotopes during the crystallisation of zircons within the

Buckland Granite is illustrated in Figure 4.10, where δ18O ‰ [SMOW] is plotted

against U-Pb zircon ages. All zircon grains sampled from the Buckland Granite

display a strong sedimentary signature, as expected for the S-type pluton. All grains

show a distinct increase in δ18O as the granite evolved, from core to rim. Outliers

within this figure include analyses B2-1 [C], B2-3 [C] and [R], B2-4 [C] and B2-5

[C], due to their variation from the overall trend. The xenocrystic cores (B2-1 [C],

B2-3 [C] and B2-5 [C]) are considered to be inherited due to their U-Pb zircon ages,

while the zircon rim (B2-3 [R]) has an older U-Pb zircon age than its corresponding

xenocrystic core due to the analysis of an unexpectedly shallow xenocrystic core. The

presence of inherited grains limits the ability to link the older cores with the igneous

zircon rims which grew within the Buckland Granite during its emplacement and

crystallisation.

Comparison between zircon xenocrystic core and rim δ18O ‰ [SMOW] values from

the Buckland Granite are shown in Figure 4.11. All data points plot above the average

δ18O mantle value, indicating a strong sedimentary component within the melt.

159

RNZ119 (all analyses)

0

200

400

600

800

1000

1200

1400

1600

1800

4 5 6 7 8 9 10 11

δ18O ‰ [SMOW]

U-P

b ag

e (M

a)

B2-4 [C]

B2-2 [C]

B2-5 [C]

B2-1 [C]

B2-3 [R]

B2-3 [C]

B2-6 [C]

B2-4 [R] B2-5 [R]B2-6 [R]

B2-2 [R]B2-1 [R]

Figure 4.10: δ18O ‰ [SMOW] vs. U-Pb age (Ma) for sample RNZ119. Arrows link cores and rims from individual zircon grains. Average δ18O mantle value of 5.3 ± 0.3 ‰ shown as the stippled area.

RNZ119 (zircon cores vs. rims)

0

2

4

6

8

10

12

0 2 4 6 8 10 12

δ18O ‰ [SMOW] (zircon cores only)

δ18O

‰ [S

MO

W] (

zirc

on ri

ms

only

)

B2-6

B2-3

B2-5

B2-2

B2-4

B2-1

Figure 4.11: δ18O ‰ [SMOW] (zircon cores only) vs. δ18O ‰ [SMOW] (zircon rims only) for sample RNZ119. Average δ18O mantle value of 5.3 ± 0.3 ‰ shown as the stippled area.

160

Histograms for samples UC08368, RNZ119, PCC06-01 and UC05592 are shown in

Figures 4.12a, 4.12b, 4.12c and 4.12d, with xenocrystic cores and rims shown

separately.

Crystallisation temperatures calculated for the Buckland Granite are plotted as Ti-in-

zircon T (˚C) against U-Pb zircon ages, to illustrate changes in temperature during the

crystallisation of the pluton (Figure 4.13). Outliers present include analyses B2-1 [C],

B2-4 [C] and B2-5 [C], which are considered to be inherited xenocrystic cores and

thus may not be directly correlated with the associated igneous zircon rims. Most

zircon grains show the expected decrease in temperature as the melt evolved towards

complete crystallisation of the pluton. The analyses [C] and [R], however, show an

unexpected increase in temperature from 800˚C to 850˚C as the melt evolved. An

increase in temperature during the crystallisation of a pluton suggests hotter,

presumably more mafic melts entered the system.

Crystallisation temperatures are compared with δ18O values for all analyses in Figure

4.14, with all data points plotting above the average δ18O mantle value as expected for

S-type granites. Most grains show an expected decrease in temperature as zircons

crystallised from core to rim. An unexpected increase in temperature is observed from

core to rim in analyses [C] and [R]. Zircon grain B2-6 appears to have a slightly older

rim compared to the core which may be due to the analysis of an unexpectedly

shallow xenocrystic core. An alternative explanation involves the replacement of

dissolving zircon by newly forming zircon in the melt, which inherits the U-Pb zircon

age of the previous grain. Crystallisation temperatures of both zircon xenocrystic

cores and rims are plotted against εHf in Figure 4.15, with εHf acting as a proxy for

161

0

1

2

3

4

5

4 6 8 10 12 1

UC08368

4

coresrims

Cou

nt

δ18O (ä SMOW)

0

1

2

3

4

5

6

7

4 5 6 7 8 9 10 11 12

RNZ119 coresrims

Cou

nt

δ18O (ä SMOW)

0

0.5

1

1.5

2

2.5

3

3.5

4 5 6 7 8 9 10 11 12

PCC06-01 coresrims

Cou

nt

δ18O (ä SMOW)

0

0.5

1

1.5

2

2.5

3

3.5

UC05592coresrims

4 8C

ount

δ18O (ä SMOW) 6 105 7 9 11 12

Figures 4.12a, b, c and d: Histograms of zircon xenocrystic cores and rims from samples UC08368, RNZ119, PCC06-01 and UC05592, respectively.


0

100

200

300

400

500

600

700

500 550 600 650 700 750 800 850 900

Ti-in-zircon T (˚C)

U-P

b ag

e (M

a)

B2-4 [C]

B2-1 [R] [C][R]

B2-5 [C]

B2-5 [R]

B2-6 [R]

B2-6 [C]

B2-2 [C]B2-4 [R]

B2-1 [C]

B2-2 [R]

Figure 4.13: Ti-in-zircon T (˚C) vs. U-Pb age (Ma) for sample RNZ119. Arrows link cores and rims from individual zircon grains.

162


0

2

4

6

8

10

12

500 550 600 650 700 750 800 850 900


δ18O

‰ [S

MO

W] B2-1 [R]

B2-5 [C]

[C]

[R]

B2-4 [C]

B2-2 [C]B2-5 [R]

B2-3 [R]

B2-2 [R]B2-6 [R]

B2-1 [C]

B2-4 [R]B2-3 [C]

B2-6 [C]

Figure 4.14: Ti-in-zircon T (˚C) vs. δ18O ‰ [SMOW] for sample RNZ119. Arrows link cores and rims from individual zircon grains. Average δ18O mantle value of 5.3 ± 0.3 ‰ shown as the stippled area.


-25

-20

-15

-10

-5

0

5

10

15

20

500 550 600 650 700 750 800 850 900


εHf (

zirc

on)

B2-4 [C]

B2-2 [R]

B2-6 [C]

B2-3 [C]B2-4 [R]

B2-3 [R]

B2-1 [R]

B2-1 [C]

B2-6 [R]

Figure 4.15: Ti-in-zircon T (˚C) vs. εHf (zircon) for sample RNZ119. Arrows link cores and rims from individual zircon grains.

163

the degree of evolution of a melt. The more evolved a granitic melt is, the more

negative the εHf value will be. All grains show a consistent decrease in both εHf and

temperature from core to rim, as expected for zircons crystallising in a cooling pluton.

εHf is plotted against U-Pb zircon age in Figure 4.16. The analysis HfB_03 from

zircon grain B2-4 is an inherited core, with an age of c. 640 Ma. The analysis HfB_01

is an outlier, but is not considered to be inherited as it is a rim. The core appears to be

younger, and is proposed to have resulted from the averaging out of ages by the

sampling of an unexpectedly shallow core. A slight inversion in age is observed

between the core (HfB_09) and rim (HfB_10) of zircon grain B2-6, with the rim

appearing older and becoming more felsic as the melt evolved

εHf values for zircon xenocrystic cores and rims are directly compared in Figure

4.17, with the TDM for the Buckland Granite being calculated using the 110.3 ± 0.9

Ma U-Pb zircon age from the LA-ICP-MS data set (Figure 4.9b). The 110.3 ± 0.9 Ma

age is statistically identical to, albeit more precise than, the SHRIMP U-Pb zircon age

of 109.6 ± 1.7 Ma calculated by Muir and co workers (1992), and is used as the

crystallisation age of the pluton for this particular figure.

εHf and δ18O values are compared for all analyses of the Buckland Granite in Figure

4.18. All analyses plot above average δ18O mantle value indicative of a sedimentary

influence, as expected for the S-type granite. Most grains show an obvious increase in

εHf from core to rim, which suggests the incorporation of a mafic component in the

later stages of crystallisation of the melt. Zircon grain B2-6 does not follow this trend,

and becomes more felsic (decreasing εHf) as the melt evolved. The greatest change in

εHf is observed in grain B2-1 from mafic to felsic εHf values of c. -1 to c. 14. No

164


0

100

200

300

400

500

600

700

-3.0 -1.0 1.0 3.0 5.0 7.0 9.0 11.0 13.0 15.0

εHf (zircon)

U-P

b ag

e (M

a)

HfB_08HfB_09

HfB_07HfB_04

HfB_10HfB_02

HfB_01

HfB_03

Figure 4.16: εHf vs. U-Pb age (Ma) for sample RNZ119. Arrows link cores and rims from individual zircon grains.


-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

-2.0 -1.0 0.0 1.0 2.0 3.0 4.0

εHf (zircon cores only)

εHf (

zirc

on ri

ms

only

)

Buckland Granite TDMB2-1

B2-6B2-2

B2-4

Figure 4.17: εHf (zircon cores only) vs. εHf (zircon rims only) for sample RNZ119. Buckland Granite TDM calculated using the U-Pb zircon crystallisation age of 110.3 ± 0.9 Ma.

165


0

2

4

6

8

10

12

-4 -2 0 2 4 6 8 10 12 14 16

εHf (zircon)

δ18O

‰ [S

MO

W]

B2-2 [R]B2-1 [C]

B2-2 [C]

B2-6 [R]

B2-6 [C]

B2-1 [R]B2-4 [C] B2-4 [R]

Figure 4.18: εHf (zircon) vs. δ18O ‰ [SMOW] for sample RNZ119. Arrows link cores and rims from individual zircon grains. Average δ18O mantle value of 5.3 ± 0.3 ‰ shown as the stippled area.

observable change is recorded in the δ18O value during the crystallisation from core to

rim in this particular zircon grain.

Zr/Hf values are plotted against U-Pb zircon ages for all analyses from the Buckland

Granite in Figure 4.19. Most analyses show the expected decrease in age from core to

rim, and decrease in the Zr/Hf ratio – with lower Zr/Hf values reflecting greater

degrees of magma differentiation. Analysis B2-4 [C] is an inherited xenocrystic core,

and therefore cannot be directly compared with the associated igneous zircon rim.

εHf is plotted against Zr/Hf values for all analyses in Figure 4.20. A consistent move

from felsic to mafic compositions (negative to positive εHf values) is observed in all

grains, as Zr/Hf values decrease. This follows the pattern expected for the

differentiation of a magma as the melt evolves. These patterns are suggestive of the

166


0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 70

Zr/Hf

U-P

b ag

e (M

a)B2-4 [C]

B2-2 [R]B2-1 [C]B2-3 [C]

B2-1 [R]

B2-4 [R]

B2-3 [R]B2-2 [C]

Figure 4.19: Zr/Hf vs. U-Pb age (Ma) for sample RNZ119. Arrows link cores and rims from individual zircon grains.


0

10

20

30

40

50

60

70

-4 0 4 8 12 16

εHf (zircon)

Zr/H

f

B2-1 [R]B2-2 [C]

B2-4 [R]

B2-4 [C]

B2-1 [C]

B2-2 [R]

B2-3 [C]

B2-3 [R]

Figure 4.20: εHf (zircon) vs. Zr/Hf for sample RNZ119. Arrows link cores and rims from individual zircon grains.

167

incorporation of a mafic component into the initially felsic Buckland Granite at the

later stages of crystallisation, as observed in Figure 4.13. The mafic imprint becomes

more pronounced during the crystallisation of zircon rims.

Crystallisation temperatures and Zr/Hf values are compared for all analyses of the

Buckland Granite in Figure 4.21. A consistent decrease in Zr/Hf is observed in all

analyses from core to rim, as expected for an evolving melt. Zircon grains B2-2 and

B2-4 show an expected decrease in temperature from core to rim, as the pluton cooled

towards complete crystallisation. Zircon grains B2-1 and B2-3, however, show an

unexpected increase in temperature with decreasing Zr/Hf, as the melt became more

evolved. The inversion of U-Pb zircon ages in grain B2-1 (Figure 4.19) supports the

inversion of temperatures, due to the analysis of an unexpectedly shallow core. Zircon

grain B2-3, however, shows no reversal of U-Pb age (Figure 4.19), which supports the

suggested heating of the system due to the influx of a hotter, presumably more mafic

component into the Buckland Granite in the later stages of crystallisation.

εHf values are plotted against Th/U in Figure 4.22. The Th/U values are consistent

within this figure, with values decreasing from core to rim as the melt crystallised.

εHf values consistently increase from core to rim, further supporting the evidence of a

mafic component in the late stages of crystallisation of the pluton. Th/U is used as an

alternative proxy for the degree of evolution of a melt, but is not always reliable as the

ratio is influenced strongly by accessory minerals such as zircon, apatite, allanite,

monazite and xenotime as U is much more compatible than Th (Hoskin and

Schaltegger 2003). The Zr/Hf ratio is considerably more reliable, as both elements are

influenced by such accessory minerals (Lowery Claiborne et al. 2007). Zircons from

168


0

10

20

30

40

50

60

70

660 680 700 720 740 760 780


Zr/H

f

B2-4 [R]

B2-3 [R]

B2-1 [C]

B2-2 [R]B2-2 [C]

B2-4 [C]

B2-3 [C]

B2-1 [R]

Figure 4.21: Ti-in-zircon T (˚C) vs. Zr/Hf for sample RNZ119. Arrows link cores and rims from individual zircon grains.


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

-4.0 -2.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

εHf (zircon)

Th/U

B2-2 [R]

B2-1 [R]

B2-4 [R]

B2-4 [C]

B2-2 [C]

B2-1 [C]

B2-3 [R]

B2-3 [C]

Figure 4.22: εHf (zircon) vs. Th/U for sample RNZ119. Arrows link cores and rims from individual zircon grains.

169

the Buckland Granite show similar distributions in both the Th/U and the Zr/Hf

figures, which suggests the Th/U values are reliable in this particular study.

4.4. Provenance of zircon grains

The geochronology of a pluton may be established through the in situ U-Pb dating of

individual zircon grains, for the timing of both primary crystallisation and subsequent

metamorphism of a rock unit. Primary igneous zircons record temperature conditions

(zircon saturation thermometry) and timing (U-Pb zircon dating) of a magma during

crystallisation. The incorporation of pre-existing zircons into igneous rocks

commonly takes place during the petrogenesis of a melt, with zircon grains often

surviving the process due to their resilient nature.

The provenance of inherited and igneous zircon grains may be investigated through

the combined approach of both geochemical and geochronological data (Hoskin and

Ireland 2000, Hoskin and Schaltegger 2003), on the basis of identification through

distinct chemical variability between potential source rocks. Analysis by LA-ICP-MS

allows the rapid sampling of several grains, for high precision U-Pb zircon dating of

events during the petrogenesis of igneous and metamorphic rocks. The comparison

between the geochemistry and geochronology of a sample with that of known

potential source rocks allows the provenance of inherited zircons to be interpreted

(Fedo et al. 2003).

Geochemical data used for provenance studies may include oxygen isotopes,

titanium and hafnium, along with the geochronological data provided by U-Pb dating.

The use of REE’s in determining the provenance of a rock unit is considered to be

limited to rocks of ‘extreme’ composition such as kimberlites (Griffin et al. 2000,

Fedo et al. 2003), and is therefore not considered further in this particular thesis.

170

4.4.1. Provenance of zircons from the PMCC

The presence of inherited zircons in the Paparoa Metamorphic Core Complex

granitoids and mylonites allows the provenance of source rocks to be investigated.

The Buckland Granite (sample RNZ119) has not been included in this section, as the

geochemistry and geochronology of the pluton primarily provides information on the

timing and nature of its crystallisation at 110.3 ± 0.9 Ma.

Geochemical and geochronological data from samples UC05592, UC08368 and

PCC06-01 have been correlated with the known provenance of potential source rocks,

including the Greenland Group metasediments and Paleozoic granitoid rocks (Figure

4.23). The Ordovician Greenland Group metasediments show two main U-Pb age

peaks at c. 600-500 and c. 1200-1000 Ma (Ireland and Gibson 1998). Paleozoic

granitoid rocks of the Western Province are dominated by the Karamea Batholith,

with an overall crystallisation U-Pb age of c. 380-370 Ma. The Cape Foulwind and

Windy Point plutons of the Karamea Batholith have younger crystallisation U-Pb

ages, peaking at c. 330 Ma (Muir et al. 1994, Muir et al. 1996).

The geochemistry of samples UC05592, UC08368 and PCC06-01 have been

established through the in situ sampling of O, Ti and Hf concentrations in individual

zircon grains, by the LA-ICP-MS technique (Tables 4.9a, 4.9b, 4.9c and 4.9d).

The geochronology of rocks of the Paparoa Metamorphic Core Complex have been

determined through the in situ U-Pb zircon dating of samples UC05592, UC08368

and PCC06-01, using the LA-ICP-MS technique. U-Pb data is visually represented in

Tera-Wasserburg concordia diagrams and probability density distribution-histogram

plots, constructed using the Isoplot Ex v.2.6. program (Tera and Wasserburg 1972,

Ludwig 1999).

171

Figure 4.23: Probability-density distribution-histogram plots of the Karamea Batholith and Victoria Paragneiss (Greenland Group metasediments), Muir et al. (1996).

U-Pb zircon age peaks are observed at c. 1000, c. 600 and c. 300 Ma in samples

UC05592, UC08368 and PCC06-01, characteristic of Paleozoic zircon-bearing

sediments deposited along the eastern margin of Gondwana, and Mesozoic granitoid

rocks of the Western Province (Ireland 1992, Muir et al. 1995).

Inherited zircons present in rocks of the Paparoa Metamorphic Core Complex are

interpreted to have been sourced from heterogeneous local crust, including the

Greenland Group metasediments and Karamea Batholith plutons.

172

CHAPTER 5: DISCUSSION

5.1. Geometry and nature of metamorphism of the PMCC

Deformation associated with the Ohika and Pike Detachment Faults illustrates that

the two detachment faults had distinctly different histories. Rocks adjacent to the

Ohika Detachment Fault display extensive cataclastic deformation, with limited

recrystallisation of quartz in granitoid rocks of the Lower Plate. Thermal

metamorphism of the Upper Plate Greenland Group metasediments in the Lower

Buller Gorge resulted in the growth of andalusite, probably in response to the

emplacement of the Buckland Granite. Offset along the Ohika Detachment Fault is

suggested to be relatively limited, as the thermally altered Upper Plate rocks are still

found adjacent to the Buckland Granite in the Lower Buller Gorge. Exhumation

along the Ohika Detachment Fault is proposed to have taken place from depths of 1.5-

5km, further supporting observations of the metamorphism of Upper Plate rocks

adjacent to the Buckland Granite. The presence of cataclastic deformation indicates

temperatures were less than c. 280˚C. Using the thermal gradient of 50-90˚C/km

(White 1994), a maximum depth of 3-6km is calculated.

The dominance of cataclastic deformation in the Lower Buller Gorge reflects the low

temperatures reached during extension, with temperatures proposed to have remained

below c. 280˚C, probably at the c. 150-250˚C range. Temperature estimates are based

on the presence of recrystallised quartz, the dominance of brittle deformation, and the

lack of observable mylonitisation of rocks adjacent to the Ohika Detachment Fault.

Deformation is distinctly different in the southern section of the Paparoa

Metamorphic Core Complex, with ductile deformation dominating the geology.

173

Cataclastic deformation is observed near the inferred Pike Detachment Fault,

followed by the mylonites and ultramylonites of Morrisey Creek. The degree of

deformation in the basement rocks increases to the north, in contrast to the pattern

observed in the Lower Buller Gorge area, reflecting the heterogeneous nature of

ductile deformation of the Lower Plate rocks.

The mylonites and ultramylonites exposed along the coastline reached upper-

amphibolite facies metamorphic conditions during movement along the Pike

Detachment Fault, with exhumation proposed to have taken place from depths of 12-

18 km. The greater degree of exhumation, along with the presence of highly deformed

rocks, supports the idea that the Pike Detachment Fault was the more dominant

structure during the onset and continuation of the Paparoa Metamorphic Core

Complex.

The onset of extension is proposed to have taken place at 116.2 ± 5.9 Ma along the

Pike Detachment, based on the Rb/Sr date of ductile deformation of the ultramylonite

(sample PCC06-02) at White Horse Creek (Ring et al. 2006). The inception of

continental extension along the Pike Detachment conflicts with earlier authors who

proposed extension initiated along the Ohika Detachment Fault to the north (Tulloch

and Palmer 1990, Spell et al. 2000).

The emplacement of the Buckland Granite resulted in the thermal relaxation of the

crust and subsequent cessation of the Ohika Detachment Fault. The dominance of

movement shifted to the Pike Detachment fault, allowing greater exhumation to take

place in the south.

174

The dip angle for the Pike Detachment Fault has been calculated using the

exhumation rate of 0.6-1.0mm/yr and the slip rate of 4mm/yr, proposed by White

(1994) and Spell and co workers (2000), respectively. The proposed dip angle for the

Pike Detachment Fault is 15-30˚. The calculated 15-30˚ dip angle supports the notion

that the Pike Detachment Fault is a regional-scale low angle structure. The presence

of large-scale low angle faults within the continental crust conflicts with rock

mechanics, as faults with dip angles of less than c. 30˚ are theoretically unable to slip

within the brittle crust (Anderson 1942). Detachment faults of the Paparoa

Metamorphic Core Complex are proposed to have been able to slip due to the

significant weakening of the continental crust during regional-scale extension.

Weakening of the crust may take place by the presence of high pore fluid pressure

along a large-scale low angle fault, resulting in brittle fracturing and cataclasis of

adjacent rock. Chloritization and sericitization may also take place through the

replacement of micas and feldspar, as commonly observed in metamorphic core

complexes (Coney 1980, Wernicke 1981a, Davis 1983).

An alternative mechanism for the existence of large-scale low angle detachment

faults involves the exhumation of the brittle-ductile transition zone (Miller et al. 1983,

Axen 2004). Low angle structures may develop within the ductile zone of the

continental crust, due to the weak nature of the crust at depth. In the case of regional-

scale extension, the crust becomes isostatically uplifted, and would result in the

transportation of the low angle structure into the brittle zone of the continental crust.

It has been suggested that faults which initially develop within the ductile lower crust

are able to remain at a low angle, as the structure remains relatively weak during the

transition (Miller et al. 1983, Axen 2004).

175

5.2. Geochemical and geochronological findings

The use of geochemistry and geochronology has allowed interpretations to be made

on the petrogenesis of the Buckland Granite and the provenance of inherited zircons

within the Paparoa Metamorphic Core Complex rocks. Correlation of data has been

possible due to the method of sampling adopted for this thesis, with analysis numbers

listed alongside the zircon grain numbers in Tables 5.1a, b, c and d.

Crystallisation temperatures of the Buckland Granite calculated using the zircon

saturation thermometry technique averaged at 697°C (3sf), with a standard deviation

of 54.6 (3sf). Xenocrystic cores and rims of zircons were calculated separately, with

temperatures averaging at 723°C (3sf) and 730°C, respectively. The temperature

difference between the xenocrystic cores and rims is considered to be negligible. The

range in crystallisation temperatures for the Buckland Granite lie within the upper-

amphibolite facies range, which is consistent with ductile deformation of rocks along

the coastline north of the inferred Pike Detachment Fault (sample PCC06-02).

The majority of zircon grains show the expected decrease in temperature from core

to rim, as the Buckland Granite evolved towards complete crystallisation (Figure

4.13). An unexpected increase in temperature is observed from core [C] to rim [R] in

one grain, which is found to represent heating of the pluton through the incorporation

of hotter, presumably more mafic melts in the later stages of crystallisation.

Alternatively, the temperature inversion may record inherited U-Pb ages of pre-

existing grains which dissolved and were replaced by newly forming zircon in the

melt. The replenishment of the Buckland Granite magma chamber is considered to be

more likely, and is further supported by the Ti-in-zircon T (˚C) vs. Zr/Hf plot, in

176

208Pb correctedZircon grain Ti analysis C vs. R Hf analysis C vs. R O analysis C vs. R 206Pb/238U age ±1s.e.

B2-5 BUCK_01 C - - 3c C 373.71906 1.88505B2-5 BUCK_02 R - - 3r R 111.1498235 0.45163B2-6 BUCK_03 C HfB_09 C 2c C 112.5138063 0.71869B2-6 BUCK_04 R HfB_10 R 2r R 135.4818137 1.05859

- BUCK_05 C - - 1c C 110.4814407 0.9017- BUCK_06 R - - 1r R 103.4311859 0.63704

B2-3 BUCK_07 R HfB_06 R 6r R 562.5694685 14.8549B2-3 BUCK_08 C HfB_05 C 6c C 1604.237201 29.2793B2-4 BUCK_09 C HfB_03 C 7c C 636.0121967 5.85876B2-4 BUCK_10 R HfB_04 R 7r R 101.3314408 0.47943B2-1 BUCK_11 C HfB_01 R 5c C 314.6349783 17.5711B2-1 BUCK_12 C HfB_02 C 5r R 116.3241816 1.71268B2-1 BUCK_13 R - - 5rr R 103.2511763 0.51961B2-2 BUCK_14 C HfB_07 C 4c C 102.7025288 0.41536B2-2 BUCK_15 R HfB_08 R 4r R 109.3607797 0.63347

- BUCK_16 C - - - - 109.4416091 0.4574- BUCK_17 R - - - - 110.6759035 0.67846- BUCK_18 C - - - - 111.7369862 0.73542- BUCK_19 R - - - - 108.060718 0.95884- BUCK_20 R - - - - 122.695243 1.32172- BUCK_21 R - - - - 118.7945414 1.28069- BUCK_22 C - - - - 550.1788569 7.68848- BUCK_23 C - - - - 470.4739599 3.9026- BUCK_24 R - - - - 108.9868464 1.03203- BUCK_25 C - - - - 1071.627117 4.7526- BUCK_26 R - - - - 1083.140186 4.67917- BUCK_27 R - - - - 101.7708905 0.49309- BUCK_28 C - - - - 276.8368327 5.1002- BUCK_29 C - - - - 228.5945607 1.84829- BUCK_30 R - - - - 77.27770481 0.77064- BUCK_31 C - - - - 239.1498642 2.94558- BUCK_32 R - - - - 301.3601198 2.97239- BUCK_33 C - - - - 520.7914135 18.4208- BUCK_34 C - - - - 103.3208177 1.51073- BUCK_35 R - - - - 101.74642 0.74715

Table 5.1a: Combined geochemical analysis details for sample RNZ119 (Buckland Granite), with zircon grain number, location of analysis (core vs. rim) and U-Pb zircon ages.

177


68-1 68_01 C Hf68_01 C 5c C 0.07608141 0.0012968-1 68_02 R - - 5r R 0.06297728 0.0006268-5 68_03 C Hf68_02 C 1c C 0.17666481 0.0017368-5 68_04 R Hf68_03 R 1r R 0.16617933 0.001968-2 68_05 C Hf68_04 C 2c C 0.0798396 0.000968-2 68_06 R Hf68_05 R 2r R 0.07119915 0.0007768-3 68_07 R - - 3c C 0.2062262 0.0034968-3 68_08 R - - 3r R 0.0741271 0.0016968-4 68_09 C Hf68_06 C 4c C 0.12545889 0.0032768-4 68_10 R Hf68_07 R 4r R 0.05230716 0.00064

- 68_11 C - - - - 0.17352587 0.00284- 68_12 R - - - - 0.15833916 0.00196- 68_13 C - - - - 0.05740094 0.00028- 68_14 ? - - - - 0.06517195 0.00157- 68_15 ? - - - - 0.081862 0.00089- 68_16 ? - - - - 0.04756201 0.00047- 68_17 C - - - - 0.19215437 0.00105- 68_18 R - - - - 0.16091985 0.00299- 68_19 C - - - - 0.21546371 0.00773- 68_20 R - - - - 0.05335202 0.00034- 68_21 C - - - - 0.10709415 0.00365- 68_22 R - - - - 0.15350531 0.0015- 68_23 ? - - - - 0.16727636 0.00121- 68_24 ? - - - - 0.08179109 0.00091- 68_25 ? - - - - 0.05836476 0.00047- 68_26 ? - - - - 0.05435531 0.00025- 68_27 ? - - - - 0.06342026 0.00041- 68_28 ? - - - - 0.0508367 0.00019- 68_29 ? - - - - 0.08962102 0.00267- 68_30 ? - - - - 0.05067122 0.0004

Table 5.1b: Combined geochemical analysis details for sample UC08368, with zircon grain number, location of analysis (core vs. rim) and U-Pb zircon ages.

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S-9 S_01 C HfS_03 C 1c C 1473.462613 6.62742S-9 S_02 R HfS_04 R 1r R 1413.899969 6.78041S-7 S_03 C HfS_06 C 2c C 1378.486856 9.21934S-7 S_04 R HfS_05 R 2r R 1203.748023 15.5701

S-11 S_05 C HfS_07 C 3c C 596.4776804 3.50282S-11 S_06 R HfS_08 R 3r R 592.8904502 1.98352

- S_07 R - - - - 352.904914 1.68762- S_08 R - - - - 610.3007943 13.4772

S-3 S_09 R - - - - 338.6017125 1.40453S-3 S_10 C - - - - 1000.848893 3.72275

- S_11 C - - - - 1170.802297 33.134S-10 S_12 C - - 4c C 995.0984611 7.52417S-10 S_13 R - - 4r R 659.88117 4.27246

- S_15 HfS_01 R - - - -- - - HfS_02 C - - 762.1101741 10.9642- S_16 C - - - - 905.0466005 5.03104- S_17 R - - - - 529.705269 4.20434

S-2 S_18 R - - - - 782.9927549 30.0136S-2 S_19 C - - - - 697.1386036 9.95564S-1 S_20 R - - - - 462.0378448 3.2312S-1 S_21 C - - - - 529.9996739 2.83876S-4 S_22 C - - - - 865.339485 14.6921S-4 S_23 R - - - - 562.758152 3.19464

- S_24 C - - - - 595.0140087 4.86137- S_25 R - - - - 381.0645092 3.74929- S_26 C - - - - 919.9552513 3.79129- S_27 R - - - - 442.6680036 2.40194- S_28 C - - - - 621.3486368 3.07765- S_29 R - - - - 486.1675665 5.77762

S-5 S_30 C - - - - 1954.576378 8.22202S-5 S_31 R - - - - 674.4936096 13.8623S-6 S_32 C - - - - 550.1452145 3.28984S-6 S_33 R - - - - 329.0483789 1.77143S-8 S_34 C - - - - 924.6954943 6.20485S-8 S_35 R - - - - 770.8304771 8.30041

Table 5.1c: Combined geochemical analysis details for sample PCC06-01, with zircon grain number, location of analysis (core vs. rim) and U-Pb zircon ages.

179


- 92_01 ? - - - - 297.4303486 3.99189- 92_02 ? - - - - 239.5477162 3.8867

92-2 92_03 C - - - - 336.0306656 2.08048- 92_04 ? - - - - 761.1094961 9.2472

92-3 92_05 C Hf92_04 C 1c C 1015.456051 4.46141- 92_06 R - - - - 160.9600057 6.35517- 92_07 R - - - - 249.3183688 13.8649- 92_08 C - - - - 665.0654357 5.52218

92-3 92_09 R Hf92_03 R - - 781.3093172 6.2477792-2 92_10 R - - - - 293.3765261 1.78448

- 92_11 C - - - - 295.2686703 2.5371692-1 92_12 R Hf92_02 R 3r R 329.0550824 5.3649992-1 92_13 C Hf92_01 C 3c C 86.64086097 1.3126692-4 - - Hf92_05 R 4c C - -92-4 - - Hf92_06 C 4r R - -

Table 5.1d: Combined geochemical analysis details for sample UC05592, with zircon grain number, location of analysis (core vs. rim) and U-Pb zircon ages.

which temperatures increase from core to rim in two of the grains analysed (Figure

4.21).

The timing of emplacement of the Buckland Granite has been further constrained

from the previous date of 109.6 ± 1.7 Ma from Muir and co workers (1992). Two

distinct U-Pb zircon age peaks are observed in the LA-ICP-MS data for the Buckland

Granite (sample RNZ119), at 102.4 ± 0.7 and 110.3 ± 0.9 Ma. The presence of two

distinct peaks in the U-Pb zircon age data may have resulted from the systematic

sampling approach undertaken for this particular thesis, where xenocrystic cores and

rims from individual zircons were analysed separately. The careful analysis of

xenocrystic cores and rims allows tracking of chemical changes within the Buckland

180

Granite as the melt evolved to complete crystallisation. This thesis is one of few

studies which presently involves the analysis of similar growth zones across a number

of grains, allowing correlations between O and Hf isotope ratios, REE, Hf, U-Pb

zircon ages and Ti-in-zircon temperatures to be made between samples.

The 110.3 ± 0.9 Ma U-Pb zircon age peak is statistically identical to, albeit more

precise than, the SHRIMP U-Pb zircon age of 109.6 ± 1.7 Ma calculated by Muir and

co workers (1992), and is interpreted as the timing of crystallisation of the Buckland

Granite. The 102.4 ± 0.7 Ma U-Pb zircon age is more difficult to interpret, but may

have resulted from three main options.

The first option is the potential influence of Pb-loss at a later stage, causing the

110.3 ± 0.9 Ma crystallisation age of damaged zircons to shift to the younger age of

102.4 ± 0.7 Ma. The second option is the remobilisation and precipitation of zircon

grains at 102.4 ± 0.7 Ma in response to a thermal event (subsolidus crystallisation), to

then become incorporated into the Buckland Granite. The third option is that the

102.4 ± 0.7 and 110.3 ± 0.9 Ma U-Pb zircon ages represent two distinct magma pulses

during the emplacement of the Buckland Granite into the Lower Plate of the Paparoa

Metamorphic Core Complex.

Pb-loss is considered to be unlikely, as analyses were carried out in grains/growth

zones showing no observable metamictization, and differences in trace element

concentrations are too great to be accounted by Pb-loss alone.

U and Th/U content, along with Ti-in-zircon crystallisation temperatures, are

averaged for the 102 Ma and 110 Ma grains from the Buckland Granite (sample

RNZ119) in Table 5.2. The 102 Ma zircons have greater U content, and lower average

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208Pb corr'd Ti-in-zircon Zircon grain LA-ICP-MS analysis C vs. R 206Pb/238U age U (ppm) Th/U T (°C)

B2-5 BUCK_02 R 111.1498235 542.3 0.13 690.4989309B2-6 BUCK_03 C 112.5138063 314.1 0.38 714.5328101

- BUCK_05 C 110.4814407 248.7 1.07 789.8182829- BUCK_06 R 103.4311859 1486.1 0.15 849.0018538

B2-4 BUCK_10 R 101.3314408 3492.2 0.02 669.902422B2-1 BUCK_13 R 103.2511763 2117.3 0.07 631.165597B2-2 BUCK_14 C 102.7025288 976.9 0.23 730.101364B2-2 BUCK_15 R 109.3607797 696.4 0.14 708.375387

- - - 102 Ma zircons (av) 2018.1 0.17 720.042808- - - 110 Ma zircons (av) 450.4 0.41 725.806353

Table 5.2: 102 vs. 110 Ma zircons from the Buckland Granite (sample RNZ119), with averages for U, Th/U and Ti-in-zircon T (˚C).

Th/U ratios than the 110 Ma zircons, while crystallisation temperatures show little

variation. Zircons grown under subsolidus conditions characteristically show high

LREE concentrations (Cavosie et al. 2006). The 102 Ma zircons from the Buckland

Granite lack high LREE concentrations, and therefore cannot be interpreted to have

resulted from subsolidus growth. A magmatic origin is considered to be more likely,

as primary oscillatory (igneous) zoning is preserved in the 102 Ma zircons (Cavosie et

al. 2006).

If the Buckland Granite 102 Ma zircons represent a magmatic pulse associated with

extension, the upper crust should display associated extensional features (e.g.

grabens) of the same age. The crystallisation age of c. 110-109 Ma for the Buckland

Granite may only represent a small event, perhaps in response to extension along the

Pike Detachment Fault. I speculate that there was a significant magmatic pulse at c.

102 Ma (Figure 5.1).

182

Figure 5.1: Schematic crosssection of the Paparoa Metamorphic Core Complex, with the Ohika and Pike Detachment Faults to the north and south, respectively. The proposed 102 and 110 Ma magmatic pulses are illustrated, with the 110 Ma pulse being dragged to the south in response to movement along the Pike Detachment Fault from c. 116 Ma (Ring et al. 2006).

Two samples of the Stitts Tuff provide crystallisation ages of 101 ± 2 and 102 ± 3

Ma (2σ error), with U, Th, Th/U and U-Pb data listed for ‘Stitts Tuff I’ and ‘Stitts

Tuff II’ in Table 5.3 (Muir et al. 1997). Sample ‘Stitts Tuff I’ has a range of U content

from 150-720ppm, and 0.47-1.29ppm for Th/U. Sample ‘Stitts Tuff II’ ranges in U

content from 200-600ppm, and 0.55-0.78 for Th/U (Muir et al. 1997).

The U content and Th/U ratios for the 102 Ma Buckland Granite zircons have been

compared with the 101-102 Ma zircons from the Stitts Tuff, with higher U contents

and lower Th/U ratios in the Buckland Granite 102 Ma zircons. A regional thermal

(magmatic?) event is proposed to have taken place at c. 102 Ma, in association with

the Stitts Tuff volcanism at 101-102 Ma.

Hafnium is a useful indicator of crustal vs. mantle influences on a magma system,

with εHf values being calculated for the Buckland Granite following the method

183

Table 5.3: U, Th, Th/U and U-Pb data for samples ‘Stitts Tuff I’ and Stitts Tuff II’ (Muir et al. 1997).

outlined in the paper by Nebel and co workers (2007). Both negative and positive εHf

values were calculated for sample RNZ119, with an average negative value of -1.7

indicative of crustal involvement in the melt (Table 4.7).

The observed increase in εHf from core to rim in most zircon grains is proposed to

record the incorporation of a mafic component in the later stages of crystallisation of

the melt, with the mafic imprint becoming more pronounced during crystallisation of

zircon rims (Figures 4.18 and 4.20). The maximum (TDM mafic) and minimum (TDM

felsic) model ages for the Buckland Granite were found to be 3410 Ma and 2969 Ma,

respectively. The maximum model age of 3410 Ma coincides with the major crustal

formation event for the Gondwana supercontinent at c. 3.4-3.5 Ga (Kemp et al. 2006),

which records the timing and location at which the inherited zircons initially grew.

184

The provenance of zircon grains within the Paparoa Metamorphic Core Complex is

investigated using the LA-ICP-MS U-Pb zircon data analysed for samples UC05592,

UC08368, PCC06-01 and RNZ119. Probability density distribution-histogram plots

and Tera-Wasserburg diagrams illustrate peaks at c. 1000, 600 and 300 Ma, which

suggest the involvement of a local, heterogeneous crustal component (including the

Greenland Group and the Karamea Batholith plutons). Inherited zircon grains in the

Stitts Tuff show a distinct age peak at c. 300-400 Ma, characteristic of the nearby

Karamea Batholith (Muir et al. 1996, Muir et al. 1997). This inheritance, also

observed in the Buckland Granite, illustrates that the two rock units were influenced

by the same local source.

5.3. Potential future work

Future work on the Paparoa Metamorphic Core Complex may involve the following

suggestions:

1. Link the silicified material in the Pike Stream area to the nearby White Knight

Stream area (resilicified Greenland Group at the inferred detachment fault,

observed during field work for this particular thesis).

2. Access drill core PRDH12 (in the core store at Greymouth) to observe the

sequence from the Upper Plate to Lower Plate (at depth), through the Pike

Detachment Fault in the Pike Stream area.

3. Try and find evidence for the suggested gravitational reversal of shear sense in

the Lower Buller Gorge associated with the switch in dominance from the

185

Ohika to Pike Detachment Fault (should see an older sense of shear to the

south, overprinted by movement to the north).

4. Systematic U-Pb zircon dating of the Buckland Granite to determine the

importance of the 102 and 110 Ma age peaks found in this study.

5. Look for any evidence of extension at 102 Ma in the upper crust adjacent to

the Buckland Granite pluton in the Buller area.

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CHAPTER 6: CONCLUSION

The Paparoa Metamorphic Core Complex was first proposed by Tulloch and

Kimbrough in 1989 to explain the juxtaposition of high grade (Lower Plate) against

low grade (Upper Plate) rocks on the West Coast of the South Island, New Zealand.

Continental extension during the Cretaceous, through the development of the Paparoa

Metamorphic Core Complex, led to the eventual separation of the New Zealand

section from the Gondwana supercontinent, and the later formation of the Tasman Sea

Basin at c. 84 Ma (Laird 1981, Grindley and Davey 1982, Bradshaw 1989, Tulloch

and Kimbrough 1989, Muir et al. 1992, Laird 1993, Waight et al. 1998b, Spell et al.

2000, Laird and Bradshaw 2004).

The Paparoa Metamorphic Core Complex was suggested to be symmetric in nature

(Spell et al. 2000), with the Ohika and Pike Detachment Faults thought to have

experienced similar histories. The nature of ductile deformation of Lower Plate rocks

was investigated, with deformation found to be distinctly different in rocks adjacent to

the Ohika and Pike Detachment Faults. The presence of mylonites and ultramylonites

adjacent to the Pike Detachment Fault illustrates the greater degree of exhumation in

the south. Deformation of Lower Plate rocks to the north is predominantly cataclastic

in nature, with limited offset along the Ohika Detachment Fault during extension.

The onset of extension is proposed to have taken place at 116.2 ± 5.9 Ma along the

Pike Detachment, based on the Rb/Sr date of ductile deformation of the ultramylonite

(sample PCC06-02) at White Horse Creek (Ring et al. 2006). The inception of

continental extension along the Pike Detachment conflicts with earlier authors who

187

proposed extension initiated along the Ohika Detachment Fault to the north (Tulloch

and Palmer 1990, Spell et al. 2000).

Geochemical and geochronological investigations involved the in situ analysis of

zircon grains from a variety of Lower Plate rocks of the Paparoa Metamorphic Core

Complex. Both internal and external morphological features were investigated, in

combination with Hf and O isotope ratios, trace metals (Ti, REE’s) and U-Pb data.

The use of cathodoluminescence images (CL) allowed any altered or damaged grains

to be avoided during sampling, thus reducing the potential incorporation of zircon

grains which have experienced Pb-loss.

Zircons from the Buckland Granite (sample RNZ119) all displayed δ18O values

greater than the average δ18O mantle value of 5.3 ± 0.3‰, as expected for the S-type

granite. The average crystallisation temperature of the Buckland Granite (RNZ119)

was found to be 697°C. Zircon xenocrystic cores and rims of the Buckland Granite

were analysed separately, with crystallisation temperatures of 723°C and 730°C,

respectively. The difference in average temperatures between xenocrystic cores and

rims of sample RNZ119 is considered to be negligible, with the crystallisation

temperatures lying within the upper-amphibolite facies range. Recrystallisation of

feldspars within the Lower Plate mylonite at White Horse Creek (sample PCC06-02)

indicates that deformation was well within the amphibolite facies range at 116.2 ± 5.9

Ma – further supporting the upper-amphibolite facies metamorphism of the Buckland

Granite during the Cretaceous.

Crystallisation temperatures of the Buckland Granite (sample RNZ119) generally

show the expected decrease as the melt evolved (from zircon core to rim). One grain,

however, shows an unexpected increase in temperature from 800˚C to 850˚C as the

188

melt evolved. An increase in temperature during the crystallisation of the pluton

suggests hotter, presumably more mafic melts entered the system. The incorporation

of a mafic component into the initially felsic Buckland Granite is observed at the later

stages of crystallisation, with the mafic imprint becoming more pronounced during

the crystallisation of zircon rims. Mafic replenishment of the Buckland Granite in the

later stages of crystallisation is proposed.

εHf values of zircons from the Buckland Granite (sample RNZ119) are both negative

and positive, with an average negative value of -1.7. The presence of negative εHf

values indicates crustal involvement during the crystallisation of zircon, as expected

for the S-type nature of the Buckland Granite.

TDM mafic (maximum) and TDM felsic (minimum) values for the Buckland Granite

(sample RNZ119) average at 3410 Ma and 2969 Ma, respectively, with the mafic TDM

value of 3410 Ma coinciding with the proposed major crustal formation event of the

Gondwana supercontinent at c. 3.4-3.5 Ga (Kemp et al. 2006).

The REE patterns for the Lower Plate rocks UC05592, UC08368, RNZ119

(Buckland Granite) and PCC06-01 all show positive trends with a relative enrichment

of HREE’s and depletion of LREE’s, as expected for zircon-bearing rocks which have

not suffered extensive alteration.

Two distinct U-Pb zircon age peaks are observed in the LA-ICP-MS data for the

Buckland Granite (sample RNZ119), at 102.4 ± 0.7 and 110.3 ± 0.9 Ma. The 110.3 ±

0.9 Ma age is statistically identical to, albeit more precise than, the SHRIMP U-Pb

zircon age of 109.6 ± 1.7 Ma (Muir et al. 1992) and is interpreted as the timing of

crystallisation of the Buckland Granite. The 102.4 ± 0.7 Ma U-Pb zircon age is

proposed to represent a younger thermal (magmatic?) event, based on the U and Th/U

189

contents, and the presence of primary oscillatory (igneous) zoning in the 102 Ma

zircons of the Buckland Granite.

The U content and Th/U ratios for the 102 Ma Buckland Granite zircons have been

compared with the 101-102 Ma zircons from the Stitts Tuff (Muir et al. 1997), with

higher U contents and lower Th/U ratios observed in the 102 Ma Buckland Granite

zircons. The suggested thermal (magmatic?) activity at 102 Ma is proposed to be a

regional event associated with the 101-102 Ma Stitts Tuff volcanic activity.

The inheritance of zircons in the Lower Plate rocks (samples UC05592, UC08368

and PCC06-01) is addressed, with three main age peaks observed at 1000, 600 and

300 Ma. These age peaks correspond with zircons of the Greenland Group

metasediments and the Karamea Batholith (Muir et al. 1994, Muir et al. 1996, Ireland

and Gibson 1998), and records the incorporation of heterogeneous local crust during

petrogenesis. Inherited zircon grains in the Stitts Tuff show a distinct age peak at c.

300-400 Ma, characteristic of the Karamea Batholith (Muir et al. 1996, Muir et al.

1997). This inheritance, also observed in the Buckland Granite, illustrates that the two

rock units were influenced by the same local source.

Future work on the Paparoa Metamorphic Core Complex may include the systematic

sampling of the Buckland Granite for U-Pb zircon LA-ICP-MS analysis, to establish

the importance of the 102 and 110 Ma age peaks observed in the data set of this

thesis. The relationship between the 102 Ma zircons of the Buckland Granite and

zircons of the 101-102 Ma Stitts Tuff is also considered to be of interest for future

work on the Paparoa Metamorphic Core Complex.

190

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---------------------------------------------------------- · – continental extensional tectonics – the paparoa metamorphic core complex of westland, new zealand. a thesis submitted